I've switched pretty much all the options (size wise) on multiple Tesoros and several widescans without encountering any problems. The one exception was the 10X12 Spider on my DeLeon which chattered a bit, but as I have several manual GB detectors I can use, it didn't matter much to me. Some years ago, I called Tesoro about this issue and was told that if I were going to use one particular coil most of the time, it COULD be a good idea to send the coil and detector in to be tuned for most OPTIMUM performance. I've had good enough performance across the board with my particular coils and detectors, I've never felt the need for that. Granted, I might not be getting the most performance possible in some cases, but I haven't seen any obvious performance problems, either. IMHO, one should always give the different coil a fair trial before deciding whether to send it off. For what it's worth, my experience/opinion comes from using 5 preset ground Tesoros which have all performed will in most of the locations I usually detect. Hope this is helpful.
BB