Here's what I've figured out after 30 some odd years with both a CAT I and a CAT II tractor on hand at the same time, sharing many implements.
It will work MOST of the time. Some implements lend themselves to being used on a bigger or smaller tractor than others. Has a lot to do with their design.
Buy the very best CAT I implements. When using "smaller" CAT I implements on a higher HP tractor than intended, you need top-shelf stuff. Don't expect the cheapest of box blades to handle what one 3 times the price will. Higher HP will destroy soil engaging equipment, PTO equipment, even the simplest of tools take a thrashing. Higher HP usually means higher weight (of the tractor) too. That equals greater traction. That equals more load on the tool hooked to that tractor.
Understand when "too much" is too much. You can always "scale back" on what you're putting an implement through. Don't expect that box blade to bury itself in hard ground just because it will. Take smaller bites.
I've had several tractors that were "just barely" CAT II tractors. (around 60 to 65 HP) Of course, they were easier to deal with CAT I implements than a tractor at the upper end of the CAT II scale. (100 HP)
Yep. It'll work, just don't be shocked when that CAT I rear blade folds up like a tent when you start pushing the same thing you always pushed with a CAT I tractor.