Yes you can still upgrade to 3.7, you just need to set the Clock on the PC your putting the upgrade CD in to a date before Jan 1 2007. I personlly though 3.7 had to many Bugs and other problems, so I only upgraded my 60 BCM's to 3.6 and they have been there for 2 years and a stable and I haven't had any problems other then Bad CAP's in the BFT.
This is a Signature and not part of the answer, it appears on every reply.
This is an Analogy so don't take it personally as some have.
Why change the engine if all you need is to change the spark plugs.