upgrade more than 50 installations to PHP 5.3

[Spirit.vn]

as web designers we had a good year 2011 with more than 50 (different) cms & other php 5.2 driven applications. Some had customizations to core as well.How does someone upgrade such amount of apps to php 5.3?Do developers of php ever thought about that?Much (popular) functions are just depreciated causing a lot of work to people like us.I really don't know how to best proceed