The corner bracket should still be in your door.
Remove the screws holding the bottom rail on.
Tap the bottom rail off.
Gently pull the rail apart enough to release the clips on the bracket.
Pivot straight up until the bracket falls or pulls out.
The new bracket prongs go into the slots you will see in the rail.
Pivot back down until the clips lock in place.
Screw the rail back on.
I don't necessarily think they are. At least, I don't think any difference in size will be enough to matter. Again, I highly recommend you remove the door from the track and look for the outer housing. I think it is very likely that your door has the outer housing still installed, because I do not know how else the pictured roller would attach without it.
If the outer housing is still installed, we will need to see it removed and combined with the inner roller portion. Seeing it still installed might not be enough.