99% of the changes affects infrastructure and/or replace old tools. Why your work should be affected by that? I migrated Seaside from 1.3 to 1.4 in 15'... and the "problem" was not Seaside-Pharo incompatibilities, but to adapt configuration to not load OB (just not choose those packages in the metacello config) Other frameworks like Magritte and Pier ran out of the box. (well... Magritte-Mophic needed a small adjust... and that was like 5') For more examples, my personal projects (I submitted 3 slides yesterday for two of them), are usually complex projects (both for web and desktop) and I migrated all of them in less than a morning work (I needed to update/modify my metacello configurations for that).
+1 Migrating to 1.4 was not really painful for me neither. The only "problem" was that i had a "startup.st" file lying next to my image that was invoked when launching the VM. In 1.4 it is loaded automatically by Pharo so it was executed twice and messed up my configuration :p I've seen more painful migration (Ruby 1.8->1.9 anyone?).