It's not quite that simple, having just done it. You need to deactivate account update and bill pay for Wachovia, change the financial institution to Wells Fargo Bank, have Quicken find the accounts there, check which ones you want to convert over and be sure to check that they are already being tracked in Quicken (else it opens a new Quicken account for you and you lose your history), and then reactivate downloads and bill pay for each account. Then you can rename the .qdf files and edit quser.ini so the correct names show in the file choice list and on the accounts.
It also doesn't help when Wells Fargo tells you they are ready to convert and it really isn't. Took over 3 days and two restores to the last Wachovia backup before it all worked. Not really a Quicken issue but shit happens and keeping the Wachovia/Wells Fargo stuff isolated reduces the possibility of screwing up anything else in the process.
So how does one merge accounts from two Quicken files without losing the any transaction and other data?