I'm curious - why would you want to migrate to a different framework?
Anyway, I think the best thing to do in such a situation is to capture the behavior of the current system in automated acceptance tests, and require the migrated system to "simply" pass those same tests, too.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
subject: Is System Requirement Specification Required for Migration Project??