My question is: Is it possible to change the Source Type when editing af source? I can't see how to do it.
Tom already answered, but let me amplify a little bit.
Almost certainly what you are referring to as Source Types manifests in the RM world as Source Templates. I do change Source Templates sometimes. But when I do, I usually do it with SQLite rather than by merging sources, simply because it's easier. It certainly could be done instead with merging.
But more importantly, when I do so I make very sure that any data from the old template also exists in the new template. Otherwise, you will lose data as Tom has already described. And generally speaking, the only way to be sure that any data from the old template is also in the new template is to use a template of my own design rather than using the built-in Evidence Explained templates.
Be aware that the conventional wisdom among experienced RM users is that when you use the built-in Source Templates, don't use them directly. Rather, make an exact copy of them (easy to do within Lists->Source Templates) and use the copy instead. That way, if you discover a glitch in the template, you can correct the glitch. The glitch could be as major as the template not supporting a data element you need or as minor as a missing or extra comma. The reason to make the copy of the template is that the copy can be edited but the original built-in template cannot be edited.