I replaced a JD 110 belt with the deck in place. Not sure about yours, but check carefully to make sure it isn't making any sharp bends, as when you would get it on the outside of a guide pin, Or around the wrong side of a bracket.etc.

Did you lift the tractor before get in to remove and reinstall the belt?

I had my JD for about 7 years and stood on my head to do things under the tractor just so I did not have to take the mower deck off. Then one day I the repair was such that I had to take it off.

I was surprised how easy it was. I don't know about the JD 125 but assume it is similar to my JD LT155 but a few years newer. There are two pins that hold the mower, and a pin on the shaft that controls the mower belt. Remove them and the mower can essentially be rolled from under the tractor.

I raise the mower put four three wheel castors under the deck, lower it on the castors, take the three pins out move it forward to disengage the front yoke and roll it out. It can be done in less than 10 minutes. It goes in just as easy. You may wish to jack up the front of the tractor to help clear the pulleys and the like.

I have tried to remove the belt without removal of the mower deck. But, it seemed hard because it is difficulty to loosen two small sheaves that are under the mower deck. So I decide to remove the mower deck. Since both of the wrench turning directions are very tight, I wonder how to determine the wrench turning direction to loosen the small sheaves, right or left? Thank you for your help.

Pulling the deck is the way to go here. You want the new belt to have the advantage of running a deck that is clean, lubed, along with blades that are sharp. It makes the job much easier too. Don't let it intimidate you.