Of course it's possible but to advise you we need to know what you want to accomplish by going down to 7 speeds, and which combos (front/rear) do you use now? We need pics of the left side of the bottom bracket to tell you what tools are needed. It does not matter if some are riveted (not welded) together, just if yours are. If so then you would either need a new cranset and possibly bottom bracket. Or no matter what you have you could just leave the front derailleur in one position. I would recommend that anyway to start, just to see if it's going to work.