While that's possible, it's not a foregone conclusion, especially since Discotek tend to be top tier for their DVD encodes too. While not always the case, Discotek often seem to do SD on BD when the source materials available from the licensor aren't of great quality, and therefore wouldn't benefit significantly from upscaling. Since you already have the DVDs, I would suggest watching an episode before double-dipping to see how they look.
The other thing to bear in mind about SD on BD is that it creates a single point of failure for a whole series. If a DVD fails, you lose a few episodes. If an SD-BD fails, you can lose 40+.