Swap iCloud account registered to an iPhone? I bought my son a new iPhone at Christmas so I now have his old one but it has his iCloud account on it. 

If I delete the account on this phone can I make a new one for me and will his account still be active and ok on his new device?
Why not set up the new phone first. Once that's done and your son has verified all the games and apps and settings are correct. Erase all content and settings on the old phone (under general in the settings app - go to the bottom where it says reset).

That way, you can set up the "old" phone with only the accounts you care about.