I've been working on a film for over two years now and it has undergone many changes. This does not affect the release forms, right? They're good for the duration of the project regardless of how long it takes or how many changes were made?
Always check with an actual lawyer, but a release is a release. There is no expectation of a time limit or deadline to complete/release a project that would invalidate the release unless they asked for one to be put in the contract or you made the mistake of including one.
You might also want to check out the guidelines if your talent is from a union as per your obligations to them (ie AFAIK, SAG actors must obtain a copy of your film - I'm not sure if it has to be within x days of completion or not)