I am not an expert, but I think I read that once you acquire immunity to a virus it might be more difficult to acquire same or better level of immunity to a similar virus.

That's why you can't take vaccine for all strains of influenza and if prediction of which strain is going to be prevalent is wrong and you take wrong vaccine you can't do much about it.

Now, the Covid-19 vaccines are different mechanism so I don't know if or how this translates. For now what I read about the way the mechanism is built on some of the vaccines is that these should really be effective even if there are large changes to the virus. But there are of course always specific mechanisms and sequences that the immune system requires to recognize and act on virus and so there can never be universal vaccine.