Hi @JesseS6,
By the sounds of it, yes, it is likely as simple as removing the drain, creating some space at the bottom of the hole so it actually sits below the surface of the concrete, and then reinstalling it.
If you were to use a grinder, there's a good chance you would hit the sides of the rebate, which can cause it to kick, which is inherently pretty dangerous.
For this reason, instead of grinding, I'd suggest using the smaller of the two chisels in this Craftright 2 Piece Cold Chisel Set. By striking the chisel with a hammer while it is held at an angle, you should be able to slowly remove a small amount of concrete at the base of the hole.
Just remember to wear gloves, long protective clothing and eye protection as some debris could get airborne.
If you accidentally take out too much material, you can always use some mortar to get your level perfect when reinstalling.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
