The honest quality note
FLAC is lossless; MP3 isn't. At 256-320 kbps the difference is inaudible to almost everyone on almost any gear — that's why those are the defaults here. Keep your FLACs as the archive; carry the MP3s.
FAQ
Album tags and artwork?
Version one carries the audio only — tag preservation is on the workbench. Your FLAC originals are untouched either way.
How fast is a big batch?
Roughly faster than the upload alone would be elsewhere. Encoding runs at several× realtime on a typical laptop.