Since the new version is mostly likely to have a superset of the old version's functionality and better decomposed to boot, you will usually want to provide a compat layer for the new one that exposes the old one's API. A few caveats: As always, be completely upfront. One important thing is to release a documentation update to the old version saying that this is obsolete and new development should target the new version. Then, point them to the compat layer and walk away.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re: How to avoid (or handle) module interface changes by dragonchild
in thread How to avoid (or handle) module interface changes by chester

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