Dealing With Theme Issues When Updating

[coert_g]

I'm hoping someone can assist me in providing the best approach to upgrading the CMS when one is using heavily customized Template/Theme files.

Historically, we've had many problems upgrading Invision Power, as our website relies on lots of custom work in the templates and theme files (including the global theme, where we need to break out of the built-in padding to implement certain design features). Our issue comes in when we need to update the CMS. We've noticed in the past that large changes to IPB (from recollection, the header was changed in a previous version, and because our custom global template (and other templates) hold different code to the updated version, there was a lot of bug fixing required.

Can someone provide me with the best approach to dealing with this? Because of the challenges, we have avoided updating the CMS for over a year, and we're starting to see users' accounts exploited. 

In short, we're worried that if we update the CMS, the templates and themes are all going to break and spit out the traditional 503 errors that occur in these situations. Resulting in lengthy and time-consuming changes to the code.

Don't hesitate to ask any questions that may help.