diff --git a/src/systems/abstractsystem.jl b/src/systems/abstractsystem.jl
index dcff371ecf..d8f026021b 100644
--- a/src/systems/abstractsystem.jl
+++ b/src/systems/abstractsystem.jl
@@ -726,6 +726,32 @@ end
     end
 end
 
+"""
+$(TYPEDSIGNATURES)
+
+Remake the system `sys` with every field replaced by the value in `kwargs`.
+
+```julia
+@variables x(t) y(t)
+@named sysx = ODESystem([x ~ 0], t)
+sysy = remake(sysx, eqs = [y ~ 0])
+```
+
+WARNING: intended for internal use; does not perform any sanity checks.
+"""
+# TODO: optionally re-call constructor to sanity check?
+# TODO: use SciMLBase's generic remake()? doesn't work out of the box, though
+# TODO: should register new tag? move Threads.atomic_add!(SYSTEM_COUNT, ...) to a separate function?
+# TODO: recreate the struct once with all new fields, instead of once for every field
+function remake(sys::AbstractSystem; kwargs...)
+    for (field, value) in kwargs
+        # like `Setfield.@set! sys.field = value`, but with `field` replaced by an arbitrarily named symbol
+        # (e.g. https://discourse.julialang.org/t/accessing-struct-via-symbol/58809/4)
+        sys = Setfield.set(sys, Setfield.PropertyLens{field}(), value)
+    end
+    return sys
+end
+
 rename(x, name) = @set x.name = name
 
 function Base.propertynames(sys::AbstractSystem; private = false)
@@ -2595,30 +2621,19 @@ function extend(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nam
         end
     end
 
-    # collect fields common to all system types
-    eqs = union(get_eqs(basesys), get_eqs(sys))
-    sts = union(get_unknowns(basesys), get_unknowns(sys))
-    ps = union(get_ps(basesys), get_ps(sys))
-    dep_ps = union_nothing(parameter_dependencies(basesys), parameter_dependencies(sys))
-    obs = union(get_observed(basesys), get_observed(sys))
-    cevs = union(get_continuous_events(basesys), get_continuous_events(sys))
-    devs = union(get_discrete_events(basesys), get_discrete_events(sys))
-    defs = merge(get_defaults(basesys), get_defaults(sys)) # prefer `sys`
-    meta = union_nothing(get_metadata(basesys), get_metadata(sys))
-    syss = union(get_systems(basesys), get_systems(sys))
-    args = length(ivs) == 0 ? (eqs, sts, ps) : (eqs, ivs[1], sts, ps)
-    kwargs = (parameter_dependencies = dep_ps, observed = obs, continuous_events = cevs,
-        discrete_events = devs, defaults = defs, systems = syss, metadata = meta,
-        name = name, gui_metadata = gui_metadata)
+    # gracefully fields, being nice if one or both are nothing
+    ext(x, y) = y # prefer sys (y) over basesys (x)
+    ext(x, y::Nothing) = x
+    ext(x::Nothing, y) = ext(y, x)
+    ext(x::Nothing, y::Nothing) = nothing
+    ext(x::AbstractDict, y::AbstractDict) = merge(x, y) # prefer sys (y) over basesys (x)
+    ext(x::AbstractVector, y::AbstractVector) = union(x, y)
+    ext(field::Symbol) = ext(getfield(sys, field), getfield(basesys, field)) # TODO: use get_...?
 
-    # collect fields specific to some system types
-    if basesys isa ODESystem
-        ieqs = union(get_initialization_eqs(basesys), get_initialization_eqs(sys))
-        guesses = merge(get_guesses(basesys), get_guesses(sys)) # prefer `sys`
-        kwargs = merge(kwargs, (initialization_eqs = ieqs, guesses = guesses))
-    end
-
-    return T(args...; kwargs...)
+    # both systems were individually sanity-checked upon construction,
+    # so it should be fine to merge their fields without further checking
+    kwargs = Dict(field => ext(field) for field in fieldnames(T))
+    return remake(sys; kwargs..., name, gui_metadata)
 end
 
 function Base.:(&)(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nameof(sys))
@@ -2634,10 +2649,9 @@ system's name.
 See also [`extend`](@ref).
 """
 function compose(sys::AbstractSystem, systems::AbstractArray; name = nameof(sys))
-    nsys = length(systems)
-    nsys == 0 && return sys
-    @set! sys.name = name
-    @set! sys.systems = [get_systems(sys); systems]
+    if !isempty(systems)
+        sys = remake(sys; name = name, systems = [get_systems(sys); systems])
+    end
     return sys
 end
 function compose(syss...; name = nameof(first(syss)))
diff --git a/src/systems/alias_elimination.jl b/src/systems/alias_elimination.jl
index fb4fedc920..ac767f8b97 100644
--- a/src/systems/alias_elimination.jl
+++ b/src/systems/alias_elimination.jl
@@ -135,8 +135,7 @@ function alias_elimination!(state::TearingState; kwargs...)
     state.structure.eq_to_diff = new_eq_to_diff
     state.structure.var_to_diff = new_var_to_diff
 
-    sys = state.sys
-    @set! sys.eqs = eqs
+    sys = remake(state.sys; eqs)
     state.sys = sys
     return invalidate_cache!(sys), mm
 end
diff --git a/src/systems/connectors.jl b/src/systems/connectors.jl
index 227b4624bf..0f0b300f99 100644
--- a/src/systems/connectors.jl
+++ b/src/systems/connectors.jl
@@ -485,9 +485,8 @@ function expand_connections(sys::AbstractSystem, find = nothing, replace = nothi
     ceqs, instream_csets = generate_connection_equations_and_stream_connections(csets)
     _sys = expand_instream(instream_csets, sys; debug = debug, tol = tol)
     sys = flatten(sys, true)
-    @set! sys.eqs = [equations(_sys); ceqs]
     d_defs = domain_defaults(sys, domain_csets)
-    @set! sys.defaults = merge(get_defaults(sys), d_defs)
+    remake(sys; eqs = [equations(_sys); ceqs], defaults = merge(get_defaults(sys), d_defs))
 end
 
 function unnamespace(root, namespace)
diff --git a/src/systems/diffeqs/first_order_transform.jl b/src/systems/diffeqs/first_order_transform.jl
index b1a51f3346..14ff5c5239 100644
--- a/src/systems/diffeqs/first_order_transform.jl
+++ b/src/systems/diffeqs/first_order_transform.jl
@@ -7,9 +7,7 @@ form by defining new variables which represent the N-1 derivatives.
 function ode_order_lowering(sys::ODESystem)
     iv = get_iv(sys)
     eqs_lowered, new_vars = ode_order_lowering(equations(sys), iv, unknowns(sys))
-    @set! sys.eqs = eqs_lowered
-    @set! sys.unknowns = new_vars
-    return sys
+    return remake(sys; eqs = eqs_lowered, unknowns = new_vars)
 end
 
 function dae_order_lowering(sys::ODESystem)
diff --git a/src/utils.jl b/src/utils.jl
index 9aa893e321..52a09bb449 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -1,14 +1,3 @@
-"""
-    union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
-
-Unite x and y gracefully when they could be nothing. If neither is nothing, x and y are united normally. If one is nothing, the other is returned unmodified. If both are nothing, nothing is returned.
-"""
-function union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
-    isnothing(x) && return y # y can be nothing or something
-    isnothing(y) && return x # x can be nothing or something
-    return union(x, y) # both x and y are something and can be united normally
-end
-
 get_iv(D::Differential) = D.x
 
 function make_operation(@nospecialize(op), args)