What you describe is pretty common for IBSers. There are many people on here who used to be C and are now D, or visa versa. Sounds like right now you might be a bit A (altnerating). You may stay alternating or move all the way over to D (by the way, alternating is sort of a weekly thing--what you are like over the course of a week; if your symptoms only change once every couple of years, then that's not alternating).
However, some people should go see a doctor when their symptoms change if they don't think they have IBS. I went through all the usual tests and nothing was ever found and I was maybe IBS with maybe acid reflux. I lived with it for several years. Then I started to hurt under my rib cage. This was a new pain location, so I saw another doctor, got a test and boom, it's my gall bladder. Well, duh, it was my gall bladder all along, but catching it in the act was a different matter. So a change in symptoms may allow doctors to finally find the real problem.
But, if you are pretty sure that you have IBS in general--as in you don't suspect a gall bladder or appendix or IBD--then I'd not worry about the change in stools unless there was blood or worse pain or you suspect that you may now have a problem with some other part of your body (reproductive organs in women can cause problems that look like IBS).