Er, if they're mp3 at the moment, how would you be removing anything further by re-mp3ing it?
I was under the impression that mp3 worked by removing the low and high frequencies that people can't hear - so doing it twice won't remove anything extra, will it?
In fact, if they ARE mp3 at the moment (which they almost certainly are judging by how quickly they stream), then FLAC'ing them creates two problems:
1. It will mislead people into thinking they're lossless.
2. You'll introduce "noise" in those frequencies that aren't present in the current mp3.