Just be sure it is available and that is what you want before you do the transfer. The vast majority of VX operated flights are many more points with MileagePlan than they are with Elevate and Main Cabin Select is not available at all. There are some good deals with MileagePlan but every flight I have looked at for my travel dates have been 20-60,000 miles with Mileageplan--especially on holiday weekends--while I could get 8,000 point tickets in MCS with Elevate or 3,000 in Y for short and medium haul flights from SFO to DEN/SAN etc.
Also as the year goes on, you can't be sure whether your flight will be operated by VX or AS.