Guys, this is not rocket science. C'mon, do I really have to spoonfeed all this to you?!
Here's how to keep as low of a profile as possible: flash your card at checkin if you have it, then check your printed boarding pass to see if they indeed changed it. If they didn't, great, you're done; if they did - or if you simply don't have your card - proceed to the next paragraph.
Once you're in the lounge (or wherever), use your computer/tablet/phone to change the FFN on United.com, from A3 to UA. Then casually ask the lounge agent/gate agent to print a new copy of your BP (without mentioning the change). Make sure he new BP has the UA number.
Capische?