I had a revision from sleeve to bypass, but it was due to complications, not for weight loss. When you have a revision, the amount of weight you lose is a lot less than the original surgery, and tends to come off slower, as well. For example... with my initial sleeve, I lost 103 pounds the first 8 months. Since I've had my revision (and it's been 8 months) I've lost 57 pounds. I'm not complaining, by any means, but it comes off a lot slower and you definitely lose less.
Also, with the initial surgery you tend to lose the most weight in the first 6-7 months. With the revision, I noticed it was the first 3-4 months. In no way do I regret it (as I said, I had some pretty gnarly complications, so a revision was a must) but it's just something to keep in mind when looking at a revision for weight loss versus because of complications.