You buy the battle chest, that's what makes you a vet. It comes with a month, after that you can play 20 sub off. Any characters you've played since 7.3.5 hit though are banking your xp and once you buy the battle chest and log that character that banked xp is instantly applied. So bye bye 20. You'd have to roll again or hopefully you haven't played them much since mid Januraryish.
If you were to buy a month 6 months from now you'd be fine, it only banks xp from a pure f2p account going to a paid account.
Also if you want to play your 20s sub on you'll have to stop xp. Make sure you turn it back on before your sub expires or you won't be able top join a battleground Q.