I have been using RVM to manage my Rubies and gems.
When I first installed RVM, the Ruby version that I installed was 1.9.2-p0. I recently installed Ruby <
You can use copy in rvm
$ rvm gemset copy 1.9.2-p0 1.9.2-p136
See more about the rvm copy command
This is the fastest way to get your gems moved over and it wont reinstall everything, just copy them over. But once you've got that squared away I'd do as the others are suggesting and start using gemsets. It's a nice way to group the gems you use in your projects.