Weight of shingles plus metal

I have a large OLD horse barn that was converted from a tobacco barn some years ago. It currently has a shingle roof but has enough damage that we need to either repair the shingles or cover with metal. I would like to put metal up but I'm concerned about all the weight on the roof if we leave the shingles. The barn is large, over 60 by 30 feet with a 12/12 pitch and when the shingle were originally put on the workers told me they could feel the whole barn moving/swaying. Granted, this was before we renovated and turned it into a horse barn where we put some new sills and new siding but I'm just on the fence about whether to leave the shingles or go the extra cost to have them removed before putting metal or I could just fix the shingles for the cheapest route but really would prefer metal. Advice???

Thank you. Yes, removing the shingles is probably the most judicious approach but of course, adds to the cost. I certainly don't want to "break the camels back" so-to-speak by adding metal on top unless I get confirmation from someone in the know that it wouldn't cause undue stress.