I'm not that old, but I remeber Debbie Dunning replacing Pam Anderson on Home Improvement. Debbie fit the cute girl that might know a bit about tools way better than Pam did. Debbie is still looks just as good today as she did then.
This happens with any long-running media in which the characters do not age in real time. Take comic strips. The Comics Curmudgeon has pointed out historical inaccuracies with Zits and Sally Forth. It's the same deal.