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:

• That API has to be perfect, including any bugs.
• You have to be willing to force people to switch if they're asking for a new feature that's already in the new version. Don't backport to the compat layer.

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 One important thing is to release a documentation update to the old version saying that this is obsolete and new development should target the new version.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.