So here is my plan. Instead of buying sugar free artificially sweetened jam or reduced sugar jelly made with extra water and thickeners, I want to take really good full-of-fruit preserves, heat it up and add my own water and thickener. I was thinking of using potato starch. Any thoughts?
Update
My goal is to reduce sugar consumption without buying artificially sweetened preserves or expensive commercially diluted preparations. My thought was to buy some really good preserves and do the dilution myself. I tried it with corn starch but it didn't thicken.
Best Answer
OK, to clear up some things here. The first thing: whatever you do, you will not have a shelf-stable product you can keep in the pantry. If you want to make that, you will have to follow a known-safe recipe with sufficient acid and sufficient sugar and proper canning procedures. It will still have lots of sugar though.
Let's assume that you don't need something shelf-stable, just a fruit preparation with little sugar in it. The expensive commercial "low sugar" jams are not diluted at all. They simply contain much more fruit than the others. So there is no way you can replicate them by starting with high sugar commercial jam.
So your options are:
Cook jam from scratch to the consistency you want. High-sugar recipes are shelf stable, mid-sugar ones need refrigeration once opened, low-sugar ones only hold for a few days in the fridge. Here you have to use the thickener suggested in your recipe.
Don't make jam from the fruit. If you want very little sugar and have to give up the convenience of preserving the whole thing, just use fresh fruit puree.
Eat commercial low-sugar jam with something else. OK, you can in principle cook a water-based pudding (that's what starch+water makes) and add the jam, but it is probably tastier to add it to pudding made with milk, or to yoghurt.
You cannot really reprocess commercial jam well. It is thickened with pectin, which cannot be re-thickened after heating. It is also sensitive to the amount of sugar and acid, so adding more pectin and water will probably produce something weird. And in the end, if you find some thickener which produces a texture you like, the product will neither be as shelf stable as proper jam, nor as sweet as proper jam, which for most people would not be worth the effort. If you really want to try, pectin should be the thickener to go.