I almost learnt this the hard way
I went Vet and let my sub ran out, but prior to this I had spent all my gold (not that I had much) on my 20 Warrior, buying lots of mog sets, all the rare two-handers etc etc, and then after being Vet for some time, I purchsed another month so I could make a Vet Huntard, and I was innocently playing on my Warrior and thank goodness I realised that I was once again earning XP. I was doing arena and having great success, too easy to be honest........ thankfully I started to question myself as to why it was too easy, then after one of the battles I realised I had earnt XP, and was like, WTF............ Then I noticed my XP bar about to tick over to 21, so I raced to SW and locked my XP.
Man I would have raged real bad if my Warrior hit 21 after everything I'd spent on her.
So yes, if unsubbed, then you re-sub, your XP Lock comes off and you need to lock it again, once only though.