Default [Q] convert r800at -> r800a

I've read some of the other forums and I am still not clear on this.

My questions is would installing r800a firmware on my r800at make it available to work beyond just on the att network. The bootloader is unlocked (via payed xda member) and I am selling my phone on ebay....I would prefer it to be "as open" as possible when I sells.

Thanks in advance