Lessons Learned with upgrading DITA-OT? (Need to move PDF output customizations to a newer version of OT)

FICO will soon be upgrading from LiveContent Architect 11.1.3 to the latest version of Tridion Docs. Tridion Docs no longer supports the old version of the DITA-OT that we run our PDF output through (version 1.5.2), so part of the project will involve porting our PDF customizations to a newer version of the OT. Because the structure of the OT has changed significantly, this will not be an easy task.

Can anyone share any lessons they learned when doing this? Best practices? Things to watch out for? Any advice at all? Thank you in advance!