well the terms of all past pre-relase packages from MSFT say that you must update to the RTM version when that becomes availible. I would not expect this to change.
they give the pre-relase a large window "just in case" there is a delay.
but in a few months when the RTM goes live you will have a legal requirement to stop using the rc bits.
I am sure some folks will hack it etc... but that's another story.