Theoretically, on a stable version you can update as much as you want because the policy is to make only bugfixes and without changing the database schema. So updating shouldn't do any harm... except if you customized some views or access rights into OpenERP and in the bad way. That's the reason why usually people avoid updating too much a running instance.

The list of update can be found in launchpad. For example for the v7 http://bazaar.launchpad.net/~openerp/openobject-addons/7.0/changes

The list of fixed issue is pretty similar: as we only apply bugfixes patches on stable version, each commit beginning with [FIX] is linked to a bug. If the bug was existing in launchpad, you have the bug number written there as well.