How to upgrade a carp pair from 13 to 14 given the pfsync protocol changed ?

I spotted this in the release notes, but am unsure how to now actual do 
this. I have two machines running carp, as a failover pair, and I 
normally upgrade BACKUP, reboot, switch to MASTER, and do the other and 
flip back.

But I dont quite see how to do this from 13 to 14 - my worry is that 
when the upgraded machine comes back up, it will just take on MASTER and 
I will have two MASTERs.

I see that it's possible to make the talk the old protocol, but then how 
do I move to the new protocol ?

thanks,

-pete.