Ok here is the deal. Right now I have a 2G unlocked/activated and IMEI changed via Ziphone. I have the $15 at&t Medianet on my account. I bought the iphone used and already had the medianet on my account so i didnt have to call them. I put my SIM in and it worked. It's been working great for a year now.
I realize if I go to the store I will have to sign a 2yr contract and add the $30 package. I asked the question that if I lost/liquid damaged my 3g iphone a month down the road could I go my old Lg2000 and add the $15 medianet back on. They yes I could.
With that being said can I put my sim card back in the the 3g and BOOM I have the $15 data plan now again?
You can, but you wont have "3G" coverage, just edge. Also, AT&T will notice eventually and make you add the 3G data plan (you cant change the IMEI on 3G).