Help LG Thrill can't get online with T-Mobile APN set up

I just unlocked my LG Thrill from ATT to T-Mobile. I have call t-mobile and have the actual APN settings written down and I change my APN setting when the T-Mobile tech on the phone with me. it just wont let me select T-Mobile after I add the new APN and I can't delet the old ATT APN so now I can't use t-mobile mobile internet but only the voice and wifi works. so any one who knwo how to fix this problem PLEASE HELP......
Thank you

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Why can't you delete the old APN?