Recasting songs will not change existing buff effects
This is for all bard and dancer songs, as far as I can tell. We tested it on dev at one point.
If you have a song effect on you and it's say, a 1-dex bragi, a higher level bragi cast over you will keep refreshing the 1-dex bragi duration indefinitely, until you walk out of the aoe and let it wear off.
This means that if you're playing a performer and cast your song without buffs, you have to then let it wear off on your whole party so you can get the actual buffed version going.
It also works the other way, if your wizard got a 160 dex version of bragi, and you have 1 dex as a performer, you can keep refreshing the 160 version with your crap one.
It makes a difference for service, because that 10 int from bless will often push past an sp-cost-threshold, and bragi, because bless + imp con 10 can be a big bonus, but sometimes you really have to cast it without, like after an almost-wipe, when the bragi tends to be the first thing ressed but the priest doesn't have time or SP for buffing you.
Fix: Check if it's changed, and make it overwrite the old song buff with the new one when the song ticks.