Greetings, as per the headline, I would like to know if it is possible to change the map of an already existing campaign. In particular, the RAF Battle of Britain campaign, which normally uses a fictional map of Finland simulating the Channel. The Channel map exists (as a mod of course), so is it doable or even worth the time to swap maps? 

Not without creating new .rds .srd and .rls files as well as tweaking some others.

There is a document laying out the process: https://forum.jg1.org/topic/7662-lonestars-create-an-external-dcg-campaign-pdf/

It's old, but a still remarkably accurate document.  There have been some new timetable commands added to DCG since it was written, but they are very specific and not necessary.