How to easily tell my school friends I changed my major?

I've been worried for a while abt telling some of my friends that I changed majors, I have about 3 of them that already knows as we've been more in touch and we've seen each other back in february so i felt like i had to tell them but I haven't had the opportunity to tell the others despite me changing majors almost 5 months ago and I kinda feel bad I've told some earlier than the others. The additional thing is we're planning a surprise bday party for a friend that actually doesn't know i changed majors and I honestly don't wanna miss out on this dinner just bcz of my incapabality of telling most of them and I also don't wanna ruin the bday vibe tmrw