I have a similar problem but minor compared to what some will be having on large sites. I have a server with both 4 and 5 but mostly 4 codebase. There's about a dozen virtual hosts that need to be got\ten up to speed.
My appraoch is is to build a second server on an old box, mirroring everything in the web root but with only php5 running and that way I'll see what will break before doing the real thing.
