The guide is a good start. but OP raises a good question on upgrading magento and bringing themes with it. of course we can always look at any reports or logs to see errors that occur. but I imagine there are functions, that will change within an upgrade as well as different calls for layouts.
This is a good question. The answer depends on how the theme was built. In v1.2 and v1.3 most of the Magento template makers just manipulated and copy and pasted the /base/default templates into their own template (XML and phtml files). We didn’t really care until v1.4 when all these themes stopped working. The reason… We have all been doing it wrong. Instead of editing and copy and pasting those XML and phtml files we should have been altering the files we needed and used CSS changes as much as possible.
There’s 700+ files in base/default. The last theme I did had 60 files (remember if magento can’t find a file it will fall back).
The general areas that should be checked when upgrading are XML updates. Your updates should only be in a local.xml file located in your theme.
I would not worry about Magento 2.0 at this point. Most likely everyone will need at least moderate changes to their themes and extensions. Magento 2.0 is still in the distance.