This is a draft (but unlikely to be improved quickly, so published just in case it might help someone out there).
First: see previous post to install Redmine 2.1.0 (just don’t execute the second half of data filling and stuff: http://beeznest.wordpress.com/2012/09/20/installing-redmine-2-1-on-debian-squeeze-with-apache-modpassenger/
Second: get inspiration from the upgrade guide here: http://www.redmine.org/projects/redmine/wiki/RedmineUpgrade
Third: *before* you call db:migrate, rename the changeset_parents table and possibly the queries_roles table to something else. For example:
rename table changeset_parents to changeset_parentsbak;
rename table queries_roles to queries_rolesbak;
Don’t forget to clean the cache and restart Apache…