slabat, get some good photos (and keep them) so you have your conditions documented and available. There are some other things at stake here. I used the same hitch you have now on my last TT without any problems. The frame mounted brackets were applied with bolts over and under the frame of the trailer. The only reason I could see for someone wanting to weld the bracket on is there was no room for the bolts, such as interference with a propane tank mount, battery box, etc. That condition should be corrected by making room for the bolt - not welding the bracket.
There are some good posts in this forum (from some time ago) you could search about hitch connections to the TT frame. People were discussing there application with forum member greenenvy, a professional hauler. Basically, they need to be easy on and off and not mark the frame when your hauling new trailers to a dealer.
If it were mine, I would contact http://www.fastwaytrailer.com/e2/hitchstyles.php
, and get their expert opinion. Also, I would contact the frame manufacturer to discuss welding on the frame in that area.
... don't know how much weld you have. If the brackets are minimally "tacked" on, they could be easily removed, ground / filed smooth, and be ready for another use. Also, check the TT's frame where the weld is.
If you can't remove the brackets without damaging them, your hitch is incomplete for re-use or resale. Also, if you decided to sell, I think most TT buyers would not want the brackets welded to the frame. It doesn't sound like you have a serious problem, and could be easily corrected with a little time and money. Which, IMO, should come from the dealer that did the welding.