Recently I migrated my project which is in development from grails version 1.1.1 to grails 1.2.1 using grails upgrade.. and it worked like charm, though there were some issues with a couple of plugins, just upgrading their versions took care of them. The problem came while submitting the forms.
we had created some forms and a few hyper links the problem was the createLink was not functioning properly,
this won’t work with <g:link> either but it works with <a> tag and <form> tag. but that of course is not a good thing to do when u have <g:link> and <g:form> available. The solution was simple
<g:formname="myForm" url="[controller:'controller',action:'action']" method="post">
so avoid using createLink if you have other options, migrating may just become a bit of a niggle if you are using createLink in g:forms and g:links.
I think the problem can be reproduced in g:remoteField and other similar tags.