OAK!Merge will update or add the data to a new record if the field has a pull down menu.  We tested with ACT 12 and OAK!Merge 12.4.2.  Data is updated regardless of whether  or not items are in the pull down menu.   Even if the field was set to “limit to list”;  it will be updated.  

Consider City and State fields.  They have pull down menus.  If your data has cities or data for the state field that is not in the pull down, it will still be added or updated by OAK!Merge.

See screen shots below for a test merge into the ACT Demo database.