Matthew, this does not look good. you have parts of the newest and somewhat experimental stuff in the databse, and there are no "downgrade" scripts from new versions into old version. ::xowiki::Form and ::xowiki::FormInstance are extensions of ::xowiki::PageTemplate and ::xowiki::PageInstance, which might be merged before the release. Both classes are very new, you will get the same problems with every other older version now as well.
The following to sql calls should most probably work to downgrade
select acs_object_type__drop_type('::xowiki::Form','t') from dual;
select acs_object_type__drop_type('::xowiki::FormInstance','t') from dual;
If not, i would say, stay for now with the head version.
-gustaf
