Hi,
It's never a good idea to downgrade nor going back and forth between versions.
instead make a copy of your project and downgrade that copy, or use Github and submodules to have your project core agnostic to the Unity version and maintain several copies each with their own Unity version to be opened with. That's how I work on the Ecosystem, for uGui, ArrayMaker, DataMaker, etc etc. It's the best approach I found for total flexibility and proper management of code source across several projects copies.
now, which platform do you want to target that you can't with 5.5?
It actually should not give you errors when downgrading to 5.3, which errors do you get?
Bye,
Jean