I have experience with 10+ year old round tins and not as much with square tins. The exp I do have with "tins with corners" is that they're more likely to dry out a bit. Round tins seem to hold up better, possibly because it might be harder to get a good seal in a corner and/or harder to maintain it. I suspect applying packing tape to a tin with an unbroken seal may help maintain the seal. However, I have remoistened aged, tinned tobacco that dried out without much trouble. Patience is important. One option is to pop the tins and store the tobak in Mason jars. There's a lot of content on this topic on this and other sites. Take your pick of experts. One seldom-mentioned point is that tins do best when they're aged in a cool, dark, out of the way spot. "Cellaring" got the name for good reason. Tins don't like jostling about and temp variations. Good luck.