http://www.docgecko.com/rails/rubygems/sunspot/2011/04/10/upgrading-and-downgrading-your-rubygems.html
Upgraded to rubygems version 1.8.1, which generates a large number of depreciation errors. This is not a major problem as all still continues to work fine, expect for a couple of my gems: the sunspot (1.2.1) and sunspot_rails (1.2.1) gem. Currently, under gem 1.8.1, I was unable to start or reindex solr.
As such, for now I decided to revert back to version 1.7.2 of rubygems. In order to downgrade your rubygems version, you have to do the following:
gem uninstall rubygems-update -v 1.8.1 gem install rubygems-update -v 1.7.2 update_rubygems _1.7.2_
And, going the other way, if you wish to update to the latest version, simply run:
gem update --system
