How to change existing site to new template?

We have a zencart site on the Pure Black theme but I am considering purchasing a premium template to improve the general feel of the site.

How can I install a new template but keep any modifications I have made to my existing template? I can't remember what modifications have been made to this template as they have been added over the course of 18 months.

Are any of the main addons eg easy populate, google merchant feed etc going to be effected by a template change?

Any help on this is much appreciated.

Download the existing site to your local hard drive and compare the file set to the original template using something like winmerge.

Are any of the main addons eg easy populate, google merchant feed etc going to be effected by a template change?

If you buy a commercial template, the answer is quite likely a big yes. Many commercial template developers screw with the core code. So much so, that they can break add-on functionality.