Moving projects with render files from 11 to 12?

[Captain Al]

We have a project where we are moving back and forth from someone with PT 11 to PT 12 ultimate, but the project seems to want to render the files again every time we move the project!
Even if we copy the render files & manually point PT to them it won't find them.
as we have a lot in elastic audio it takes forever to load project.
Any tips on how to keep them easier to sync?
We are using Dropbox to transfer the songs could that be the issue? We are also copying the project to saving a copy as, we've don't this before with no problem, once the audio files are the same we should just be able to swap the project file?
Appreciate any help