you should be able to switch them without any problems that i can see. the pci bus on your mobo runs at 33 mhz and when you raise the front side bus from100 tp 133 you change the speed your pci bus runs at, at 133 mhz fsb your pci bus would then be 44 mhz and very few pci cards can run reliably at that speed. now with the rev 1.1 you have a 133 mhz fsb setting that gives you a 33 mhz bus with a setting of 1/4 rather than 1/3.
So if you can talk your girl into letting you hack her system then switch boards. one rule though spend the time to put it back together right. no missing screws!..............lol......................
I hope this helps, oh and your tnt 2 card should be ok with the higher than spec agp bus.
And as for selling the mobo, if you can switch them I WOULD NOT sell it. the bh6 rev 1.1 is an awfully stable board for overclocking and all around one of the best boards abit has ever made, it just does not have all the fsb settings most newer boards have.
Mine is more stable than my be6 with my pIII 700 at 933