Not sure if you're a Mac user or not, but you have to turn iMessage and FaceTime off on both Mac and iOS devices before switching you SIM. If you don't have your iPhone anymore you can use this site to de-register your number.
https://selfsolve.apple.com/deregister-imessage?linkId=14271830
If you've already done all that it's a good idea to have all your iOS using contacts to delete all your old message threads on their devices and start over.
If you have an iPad or Mac desktop you can then turn iMessage back on and set your email as the contact preference to send from. Just have your iMessage friends add your email along with your phone number to their contacts list. That way you can still technically use iMessage from you iPad or Mac without having your phone number attached to it.
Edit: Sorry didn't see your last post. Yeah, group chats are a pain to get working right for some reason. I've always had problems with those.