Hi Christine,
You may have already started doing the Grid Edit method at this point but another option is to use the Find and replace in these records option on a tables List All report search for the old related record ID to replace it with the new one. It can help speed it up a bit compared to using the Grid Edit since they are bulk changing. You just want to make sure you use a report that captures all the right records.
------------------------------
Evan Martinez
Community Marketing Manager
Quickbase
------------------------------