How do I swap the Trainer battle theme with the Battle Subway theme? (BW2)

[AvatarMew]

I'd like to swap the trainer theme with the battle trainer theme in for randomised Black 2 roms.

Whenever I use the universal randomisor it changes the file size from 512MB to 274MB which means I can't use the trainer battle theme modifier cheat.

I'v been trying to use 

to swap the music but I'm not sure how to do it.

I've extracted the music file.

I've tried to use Crystaltitle to swap the hex values of the trainer theme and subway theme.

127-SEQ_BGM_VS_SUBWAY_TRAINER is the music file of the Subway music, 126 is the normal battle theme.

From what I know I need to change the values and find the offsets but I'm not sure how to find them.

Edited June 17, 2019 by AvatarMew