Switching rails versions 2.0.2 <-> 1.2.5

I’m migrating my web app to 2.0.2 . Since I needed to switch back to
1.2.5 I set RAILS_GEM_VERSION in evinroement.rb:
RAILS_GEM_VERSION = ‘1.2.5’

But restarting the webserver I get an error related to the 2.0.2
version:

/usr/local/lib/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/base.rb:

So it seems that setting RAILS_GEM_VERSION did not do the trick…
Any clue?

Thank you in advance.