Yes , It is useful to us that create new Requirement document , test planning report and test cases for a "Migrating Project " because it helps us to develop our product understanding and celerity towards product .
Actually it will depends on many aspects. Like-
Is this projects gone through proper testing cycles during previous releases? if yes then surely there will test cases, requirements documented somewhere. We need to just use them.
Second thing is, is there any enhancements or updates? If yes then surely we will need to capture requirements, write test cases for these enhancements or updates etc.
In all cases if we are testing the project we will surely need test planning.
Most imp thing is, we have proper time to do all these things?? Generally In Migrating Project, testing teams are involved at last movement for just to check out Allll is welll or not?
This is the wost thing happen to testers lack of time & mgt support.