Weechat's implementation circa last July left a lot to be desired as it couldn't even federate on its own network - messages I received on my iPad were not present on my Android! Perhaps they've addressed this since but (not to be deliberately contrarian) seamless is far from the adjective I'd use; indeed they seemed miles behind their Western contemporaries and more in the league of Line (another large Chinese messenger application for those unfamiliar). I understand they have a huge Asian user-base so there's presumably a gravity factor, but it certainly didn't seem like they had it together at all.
If you are permanently changing your phone number, and want new people to be able to send you messages, then I think you need to follow the process here: https://www.whatsapp.com/faq/en/android/27585377