Quality is initially determined by parsing the filename. If it’s not in the filename, drone defaults to the extension, which is SDTV for mp4. (There is a trello card about implementing media info for that purpose)
Please note that almost all scene releases have quality in the filename, so if you don’t that’s probably due to the naming scheme you had in sickbeard.
If you have an old episode in an old series you’re happy with, but want higher qualities for new episodes. unmonitored them. If it’s the same quality as you want for new episodes you’ll need to reconfigure the quality profile.