How to authorize my iPhone to my old apple account to transfer my music?

I don't want to lose my music because i have over 200 songs on my old iPod. My mom and I can't find out how to authorize the iPhone ( I own a iPhone 4S) to the old account so that I can Keep the songs and everything. So this is basically what i want to do:
1.keep all my songs
2.keep the apps
3.transfer BACK to my NEW account without losing anything
4. Be able to listen to music without any issues

Download iTunes open it up and go into the store. Then there's a setting in their that will allow you to activate and deactivate your other computers. Then it will allow you to right click your iPod and transfer purchases.