Redoing a shed roof, it's decked with 50 year old or so plywood, tore the shingles off, bottom 3 feet or so of the ply is bad, thinking of just replacing the bottom 4 feet of ply since ply is so expansive, but having second thoughts, going to do ice an water shield then architectural shingles. Should i just replace all the ply? I think the current decking is 1/2" and the slope is 12/12
If it's good where you cut it at 4 feet, and you're going to keep it dry, no reason to change out good plywood, especially at current prices.
It's not bad due to age, it's bad due to getting wet, so if it didn't get wet enough to go bad higher up, there's no reason to replace it.
Remember to use proper drip edge, among other things. Lack of that might have been a contributing factor.