With mercurial or git really easy, with svn it might be really easy :)
I have been lucky, having to merge fixes from the uDig trunk into the uDig 1.2.0 release tag, which, as it seems, was not really ready and didn't pass the QA.
Few steps to do the merge:
1) checkout the tag into which the new changes have to go
svn co http://svn.refractions.net/udig/udig/tags/1.2.0 udig1.2.0tag
that creates a folder udig1.2.0tag with the tag code in it.
2) cd into that folder and check for the differences with trunk:
svn diff http://svn.refractions.net/udig/udig/trunk -r31936:31952
31936:31952 are the revisions that were added to trunk after the tag.
Check the changes and see if the patch can work out for you or.... if you nothing scares you....
3) do the merge! :)
svn merge -r31936:31952 http://svn.refractions.net/udig/udig/trunk
Well, if you are lucky you have no conflicts, else just open up your IDE and fix them. Then commit. Your merge is done. Hmmmm, why is this always so scary?