How can I upgrade the ruby interpreter without losing my gems inside my gemsets?

Use rvm upgrade

Say I want to upgrade from ruby 1.9.3 to ruby 1.9.3-p286 from within rvm and not loose my gemset environments.  I would type the following command.

rvm upgrade 1.9.3 1.9.3-p286

Then you just follow the command line prompts that come up, simple as that. =D