Originally Posted by
hardworking21
Ah gotcha, perfect! And once it's done as part of check-in, the issued boarding pass should show TSA Precheck?
I'm guessing this would also be the best way to update my FFN from UA --> SQ (during the check-in process, that is)?

TSA Precheck should show up on your boarding pass once you have entered your KTN, and the details of your reservation match with your TSA Precheck information.
You should also be able to update your FFN during OLCI as well. If not, you'll have to do it at the airport.