UPDATE: Not knowing how the forum worked, I asked a question and, in retrospect, I should have provided more detail on work I had already done / was doing when it occurred to me to ask the question! With very kind intention, my post was edited to ask what people assumed would be easier. However, I've already spent ages scanning the old card fronts and purchased the card sleeves for how I'm resizing the cards, it really wouldn't be easier (or definitely not cheaper) to do anything other than to press ahead at this stage.
An old favourite game (the Judge Dredd Board Game if anyone remembers or cares) has gotten to the stage when all of us can recognise half the cards from the back by the staining and marks on the cards. This is the second time I've bought the game as it happened before as well.
I had considered re-buying again and getting deck sleeves but (perhaps erroneously considering answers given) I didn't believe that I could get the right size of cards. So I have scanned all the different sets of cards (this afternoon since posting), redesigned the card backs (less solid black) and have resized the cards slightly to perfectly fit the card sleeves that I have already ordered.
I only really want to know if this world work with a lightweight card (say 150GSM) or whether something like 270GSM would work better.