Hi all, perhaps I can help. My family has 3 iphones, two ipads, and two Touches. we also use iMessage ratherthan having an ATT texting plan, so needless to say we've experimented with this quite a bit. and the good news is there's a LOT of flexibility to how you can get different messages going to different devices. So first to answer one big question, which is:
"how do you change the icloud if you already entered your apple id on all of your devices??? ahhhh!!"
For iMessage specifically, go into SETTINGS - MESSAGES. and turn iMessage OFF. Then turn it on again. Once you do, the next box down says "User your AppleID for iMessage". Click that box, and you can change the appleID if you wish.
Okay next, the way to get different messages going different places is by using the "Send & Receive" box a couple of spaces down in the settings window. There, in each device, you can choose to which phone numbers and email addresses within that iCloud account you want messages to go to. For example, my phone receives messages to my cell number and my primary email address. So does my iPad. My wife, however, only wants texts to go to her phone, so she has her phone number only listed on her phone. On her iPad, she has an email address listed which is handy when I know she's on her iPad and I want to text her.
One last thing. In the Messages settings there's a box entitled "Send as SMS" and another labelled "MMS Messaging". If you don't pay for a texting account, turn those off so non iCloudians cannot text you and run up your texting bill.