I’m sorry to hear about your diagnosis, but do understand what you mean about the relief - I vividly remember (and will always remember) being where you are just over four years ago, and I was almost elated for the first couple of weeks, because if I wasn’t physically ill with anything, it was starting to look as if my problems must be psychiatric in nature, which was not really much comfort!
I think most people with RRMS do eventually go on to Secondary Progressive (SPMS), but it’s not inevitable. I think the conversion rate is something like 80%, and the timescale typically more than ten years, but I haven’t bothered to double-check my figures, so do bear in mind my memory is not what it was!
What may be confusing you is that (unless they were misdiagnosed) somebody can NEVER move from RRMS to primary progressive, or vice versa. Those two are mutually exclusive: if you have primary progressive, you have it right from the start, and will always have it. It’s not something you develop after many years of RRMS, and it can’t change into any of the other types either.
Typically - but again, no guarantees - relapses are most frequent in the early years, and lessen over time - not increase. In other words, it gradually changes from an episodic nature to a continual, but less bumpy, decline. Not surprisingly, most people are diagnosed when relapse activity is, or has recently been, quite high - as something must have happened to spark the investigations! Over time, there tends to be a decline in relapses, but conversely, an accumulation of disability.
None of this is set in stone, and the boundary between RRMS and Secondary Progressive is not that black and white. In theory, a diagnosis of Secondary Progressive would mean someone did not have relapses any more, but as the MS does not know it’s been “redesignated”, it doesn’t know it is not meant to chuck in the odd relapse. So it’s not unheard of for someone who was thought to be Secondary Progressive still to have some, but less frequent, relapses. It’s more of a continuum, not a switch, if that makes sense?