Normally you can create a 'Child' of the style that you are using and set that as default.
Any changes you make, you do so to the templates within the 'child'
If you have to upgrade the style at all then it's just a case of uploading the upgraded style and overwrite the parent, the child should adopt the changes.
For some reason this didn't quite go to plan, perhaps it was because it was such a major upgrade and a fair bit of code changes