You either have to manually edit your client letters (not recommended) or wait for the Lacerte program updates.  This may take awhile.

What I'd probably do is add a "note" (Screen 47, Letter Misc 2) that the deadline has been postponed to May 17, that the date above is software generated and has not been updated yet, etc...   What you say and don't say would depend on the savvy and volatility of each client.

This way it doesn't mess with your client letter templates for when the change finally gets rolled out.