How to modify NHibernate mappings at Runtime

When you have the need to modify or change your NHibernate mappings at runtime the obvious choice would be to go for one of a new generation mapping engine (like ConfORM, Fluent NHibernate, or anything else), but what about you plain good old XML mapping files you still use? Changing them was a nightmare before NHibernate 3...I gave up on the task when I thought I had to pre-load the XML files,