Do you still need help with this? It's sad to see how some people never get help around here D:
Anyways, are you doing this for the purpose of playing on the stage with changed music offline or online? If you don't care about having the other people hear the same music online, then you could use a BRSTM file of the Spear Piller song and put it over H01 or whatever song you want to place it over on Pokemon Stadium. If you don't want to do that, then there is a long list of music modifiers on
http://www.geckocodes.org . I'm pretty sure these change the music of the specific song into a different song with hex values for the different song ID's (but only songs already on Brawl, obviously). I once used music modifiers (perhaps more simplistic ones then on that site) specifically for online play, but eventually I found out it was host only.
If you were to switch the whole track around, in some cases it would not work, as some stages have more/less songs, and you know the first, complex music mods really could screw up your game. I figure instead of switching whole tracks, you could replace specific songs with the ones you want, using any of the above methods. Please correct me if I'm wrong, but I'm not sure how else you would want to accomplish switching songs.