I modified a ceiling fan support box to fit onto a ceiling fan support brace. I did this because I wanted a ceiling fan in my dining room, but the junction box had three conduits running into the top of it. It would have been way too much effort and/or money to try and move or replace the conduit with flex to fit the original box that came with the fan brace.
The box you used is slightly smaller (20 cu.in. vs. 21.5 cu.in.), but that won't matter as long as you still comply with the box fill rules. It also doesn't appear to be labeled for fan support, which means it can only support a fan up to 35 lbs. Aside from those issues, I'd say there's no problem with what you did.