BB Curve 8900 :: OS - Unlock The Different Vendor Themes

I am new to this forum and new to blackberry in general. I actually purchased my very first one saturday. I bought it used because I have big hands and did not know how well my fingers would work on the tiny keypad. I have a few questions. Please dont flame me because I have been searching and reading since i got home yesterday afternoon.

1.) This phone is a t-mobile phone and is currently unlocked and works with my AT&T service. Being the engineer that I am- Its hard for me to accept the t-mobile screen that comes up on start-up. Before I call and get my blackberry plan activated on my phone I would like to have all the bugs worked out. I would like to somehow make this phone think its an AT&T phone. I have downloaded crackmem and nuked this phone. I upgraded to the lastest version of AT&T software which is version 4.6, but when i power this phone up it still shows T-mobile. That drives me crazy.

2.) I read how to unlock the different vendor themes on this forum. I tried it and it did not work for me. I entered the vendor id number for AT&T. maybe I should have entered it for t-mobile. Like i said I am new and know dont know very much about these phones.

3.) I also have the OS system 5.0.411 downloaded and ready to install on my phone. I would like to figure out how to resolve the above items before I install in on my phone. I chose the 5.0.411 over the latest software because i like the unlock button on the top of the phone working. Its just convienent for me.

My main concern is I was the t-mobile start up replaced with the AT&T one. I know it can be done but I am a mechanical engineer not a software engineer and it was hard enough to write enough code to get through college, much less reprogram a phone.