change phone number associated with apple id

Why can you not change the phone number associated with an apple id? I recently bought a new iPhone 5s and gave my 4s to my husband. When he put his sim card in the 4s, it seems like a message came up asking if he wanted to use that number for iMessage and I clicked yes never knowing it was going to change the phone number I have associated with my Apple id. He is not tech savy at all and we both share the same Apple id. I have always had my phone number associated with it and so when I get an iMessage it comes on my Mac computer, iPad, and iPhone--which I like. Now my computer and iPad only have his number with it and there is not an option to add my number back. My phone still shows my cell number associated with iMessage. I am afraid to update my phone in case it knocks me off of iMessage. I have tried the signing in and out of IMessage and Face time on my computer and iPad and that does not help. I just want to change the phone number associated with it.