PHP3 & 4

Hi folks, I have a friend who administers 150+ Linux boxes for a hosting company, and he's having a fair time trying to figure out just how to go about upgrading without breaking a lot of code written in PHP3, particularly when it comes to the GDLibs . . . Since the newer GD has no gif support, what are the options? Running 3&4 side by side? Sorry I'm so skimpy on details, but I'm getting this second-hand. =0( Also, there seems to be a dearth of info on what libraries are depended on for the "add-ons", like crypt, etc Any pointers in the right direction would be greatly appreciated! =))