As I understand it, this coronavirus, like others, is actually quite stable. Some strains may be more vicious than others, but once you’ve had one, you’d likely be partially immune to most of the other ones.
It’s an RNA virus, so stable is relative. But mutations that affect a virus enough to make it unrecognizable to your immune system are rare. It would have to occur in a part of the virus that is “tagged” by your immune system but without breaking the virus’ ability to attack your cells.