Migrating ATC 1.1 to ATC/ATE 4.1.1

I am looking to any experience/problems encountered during any migration from ATC 1.1 to ATC/ATE 4.1.1

It seems that there are some slight differences on how these two versions work that my cause code writen for 1.1 not work properly with 4.1

Thanks for any feedback and apologies for ant cross posting.