Well I haven't changed my number since that free WhatsApp promo they had back in BBOS days. And guess what, what ever phone I have my SIM card in, it has always a life time of WhatsApp license.
Well try it, if it actually stops and u actually need it then pay.
But I highly doubt u'll have to!
I can't wait till whatsapp becomes a fully paid service, am sure many will jump to bbm!
I loved whatsapp as a cross platform thingy, but hated the whole phone number thing.
With truecaller, whatsapp, and Internet connectivity (ie unlimited messages) combined spammers, adds, even (and u have no idea how many) prostitutes and scam artists keep bombarding my phone! A headache! One of my friends actually has 122 blocked contacts!
Fortunately I have a lifetime subscription and don't plan on changing my number anytime soon.
If they ever want to charge me: I'll ditch WhatsApp and fully switch to Facebook Messenger as even now almost nobody is on WhatsApp (and don't get me started with the problems I have using BBM ).