Upgrading and downgrading documents

[lumjjb]

From: #128 ... Should be partially addressed by #152

• Upgrading and downgrading documents: It could be useful to have a function that e.g. takes an SPDX-2.1 document (spdx.Document2_1) and converts it to an SPDX_2.2 document (spdx.Document2_2). Or vice versa, going the other direction.
  • This would likely require converting the various sub-elements to the new version.
  • Could also take a strict parameter to fail if the conversion can't be done with 100% fidelity. For example, if an SPDX-2.2 document has a field or value that would be invalid in SPDX-2.1, then if strict is true then the downgrade from 2.2 to 2.1 should fail. If strict is false, then the downgrade would silently (or with a warning) drop the invalid data.