This could be done with a query (faster) or VBA - it is your choice.
To duplicate the table (make a back up), right click on the table name and select "Copy". Then right click and select "Paste".
Your query would looks something like:
Code:
UPDATE Table1 SET Table1.ColA = Table1.ColB
WHERE Table1.ColC ="U";
Change names as appropriate.....
VBA would take longer because you would have to step through each record (50,000) of the table.
Warning: Make a back up of the database BEFORE trying this!!!! You have been warned......