Yep basiclly you have 2 options...
One:
- Upload old xmls view the differences between each
- Take note of the custom coding (cut and paste in nodpad or something)
- Revert old template (this way you know all the new stuff is there in the template)
- Open reverted template and add the custom coding from old
two:
Create a new default style and add the custom coding and css from old templates.