Ruby will allow you to require gems installed on a different version managed by
rbenv, however when you go to uninstall that Ruby version you will need to
re-install all of your gems. This little script transfers them automatically;
just run it from the Ruby version that you want to migrate to, and pass in
the version you wish to migrate from as an argument. It will check to see
which of those gems you don't have installed in your current version, and
transfer them if compatible.
For example, to upgrade Ruby 3.1.0 to 3.2.1:
rbenv install 3.2.1
rbenv local 3.2.1 # set Ruby to target version
rbenv-migrate 3.1.0 # pass the old version as an argument
# all of your compatible gems from 3.1.0 will install to 3.2.1
rbenv uninstall 3.1.0 # now safe to uninstall