If HipHop supports 99% of PHP 5.3, then it took them months to change 1% of their code. Changing any more than that would have obviously taken longer, especially to a whole other language.
Stuff like closures, eval(), create_function(), constants with dynamic names (e.g. defining constants from an array) and class destructors were all not supported. Some more subtle stuff: you cannot use current() to get the first element of an array in Hiphop, class_exists() always returns true.
We also had to port our custom PHP extensions.
Finally, we spend a lot of time on related stuff such as precompiling all Smarty templates (so that the generated PHP gets translated to C++ too), using Hiphop's daemon system and setting up compile, build and deploy systems.