There are a couple of options when you upgrade to 1.9.3. Since you have rvm, you can have multiple versions of ruby.
I started off with 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin11.2.0]. My options were to install a new version of ruby and along with it update all the gems I have downloaded. I did not want to do that. I just wanted to install a new version of ruby. If I needed any gems, I wanted bundle install to install it as needed. This would help me reduce the clutter.
So I tried the command rvm list known. It showed me that the latest ruby was 1.9.3.
> rvm upgrade 1.9.2p290 1.9.3-head
ERROR: The source ruby was not a valid ruby string.
Then I tried.
rvm install 1.9.3-head
After a while I got:
ERROR: rvm requires autoreconf to install the selected ruby interpreter however autoreconf was not found in the PATH.
All I had to do was this:
rvm reinstall 1.9.3
It worked like a charm.
Here are some commands I used after that.
> rvm list
=> ruby-1.9.2-p290 [ x86_64 ]
ruby-1.9.3-p0 [ x86_64 ]
> rvm –default use 1.9.3
> rvm list
ruby-1.9.2-p290 [ x86_64 ]
=> ruby-1.9.3-p0 [ x86_64 ]