backwards compatibility

In short, no.

In longer, why on EARTH would you want to run on an older version?
You're asking for trouble.

--Matt JOnes

Well, i havent the need to go backwards, but testing out xlsuite I have
tried to get it running with the code from git and the bundled rails
wasnt working well. I ran on Debian, rails 2.3.3,ruby 1.8.7 and xl is
on 2.2.2 but I couldnt get it working so hence the reason to ask. I did
remove the newer version and installed 2.2.2 to get it to work but it
was complaining about encoding issues...

maybe it wasnt the best idea but a lazy thought...

You don't need to remove a later version of Rails to use an earlier
version, just setup the version you want in environment.rb, for
example
RAILS_GEM_VERSION = '2.3.2' unless defined? RAILS_GEM_VERSION

If you want to freeze that version into your app then you can do
rake VERSION=2.3.2 rails:freeze:gems

Colin