What Should I do? Messages Help.

I have the phone number and am connected with some acquaintances from Sixth Form who I was on good terms with and we chat here and there, usually just as a catch up.
Even though we aren’t close, it’s very nostalgic for me because it brings back memories of my school years.
I’m going to get a new phone soon. But how can I get them to add my new phone number for messages/as a contact?
It’s complicated as the only reason we still have messages is as a relic of school when we added each other and we obviously wouldn’t just randomly add each other now since we haven’t spoken since so they might think it’s weird to have the phone number of someone they will never see.
But I just want to continue to send them “Happy Birthday” messages or receive it from them and occasionally send messages.

I have the phone number and am connected with some acquaintances from Sixth Form who I was on good terms with and we chat here and there, usually just as a catch up.
Even though we aren’t close, it’s very nostalgic for me because it brings back memories of my school years.
I’m going to get a new phone soon. But how can I get them to add my new phone number for messages/as a contact?
It’s complicated as the only reason we still have messages is as a relic of school when we added each other and we obviously wouldn’t just randomly add each other now since we haven’t spoken since so they might think it’s weird to have the phone number of someone they will never see.
But I just want to continue to send them “Happy Birthday” messages or receive it from them and occasionally send messages.