I was checking customization upgrade subject since we faced it and I found this interesting note (451934.1)
The note is talking about how to make custom forms that where created and working fine on 11i to work the on R12.
And according to the note
"All custom forms that were build and working fine on releases 11i are designed and compiled using the Form Builder 6i, while the developer version for R12 is 10G.So you need to open the custom forms in Form Builder release 10G and compile them and save then upload them again."
I did not try the solution yet but thought of sharing it since many were asking bout this subject lately.
Also i found the (Custom forms does not work after upgrade to 12i) forum helpful