It doesn't happen always, but in most cases, an update changes the syntax of some actions, which makes the flow incompatible with previous versions of PAD.
This happens even if you don't make any changes, but if you open and save the flow via a later version.
If you work in a team, you do need to make sure that the entire team uses the same version. If possible, update your instance to the latest version.
If the latest version does not work for some reason and you must absolutely downgrade to a previous one, the only two ways of doing it are:
1. Rebuilding your flow from scratch.
2. Updating to a later version, then copying all of your actions to text files, then downgrading and attempting to paste the code back to a new flow. Some actions will not be accepted due to changes in syntax, but then you won't need to rebuild everything. See here on how to copy PAD code to text files: https://www.wiseowl.co.uk/blog/s2989/copy-flow.htm
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