Changing an occurrence of a meeting series (known as creating a broken-out
appointment) should work without problems.  Having said that, my experience
is that if you have a long-running series and there are lots of broken-out
occurrences over time, the chances of the meeting item getting messed up just
get higher and higher.  For that reason I try to have my meeting series end
after no more than a year, and even less for ones that frequently get amended.

But maybe someone else knows some tricks to make this more reliable...