It is a clever solution to the design bug. Probably like many other readers, I'm trying to think of the underlying problem.

My 3rd thought was to see if all the DMO interfaces to the tables are through stored procedures. Then in a pinch you could always do the "If the customer is contained within the new group customer table, SELECT the data from there, otherwise get it from the appropriate individual table."

That way, you could theoretically migrate the tables a few customers at a time. Do a pilot run with a few customers to vet out any potential bugs, and then plan the migration. Since its dynamic SQL, you can slowly implement it to avoid the BIG BANG, if you want.

Ok, punch holes in this one.