I had Rails 2.0.2 before i started my upgrade. When i executed the typical “sudo gem install rails”, i got the following error:

Bulk updating Gem source index for: http://gems.rubyforge.org/

Updating metadata for 13 gems from http://gems.rubyonrails.org/
………….
complete
Bulk updating Gem source index for: http://gems.rubyforge.org/
ERROR:  Error installing rails:
        invalid gem format for /Library/Ruby/Gems/1.8/cache/activesupport-2.1.0.gem

Steps to fix this:

• Remove all old activesupport gems from your Library
  sudo rm -rf /Library/Ruby/Gems/1.8/cache/activesupport-2.1.0.gem
  sudo rm -rf /Library/Ruby/Gems/1.8/cache/activesupport-2.0.2.gem 
  sudo rm -rf /Library/Ruby/Gems/1.8/cache/activesupport-2.0.991.gem 
• sudo gem update
• sudo gem install rails