We currently have the tractor with a mechanic....The mechanic is afraid if he splits it open that the clutch pack will come out in pieces.
You might consider a different mechanic. That's not meant to be snide, but if the Powershift assembly is damaged sufficiently that it "comes out in pieces" that cannot be assembled, it is either physically broken or the mechanic is inexperienced and unfamiliar with this transmission system. I'm not familiar with it either, but the basic principles of operation are relatively straight forward. The gear shift controls a valve that routes fluid to the different clutch packs that engage, under hydraulic pressure, to select each different speed.
If the hydraulic system is compromised (Through contaminated fluid, corrosion/oxidation, clogged filters (Probably two on your FX255D) or a stripped hydraulic pump coupler, then it is possible the packs will not be sufficiently activated to function properly. If the shifter valve seals leak a similar event could occur, as could the relief valve for the Powershift system; apparently they are known to have hung open on a contaminant.
The diagnostic procedure for isolating Powershift issues is in the service manuals for the US market machines; I imagine the YM276 would be similar enough to be useful. While speculative (Since I don't have a manual), I would begin with removing the suction screen and thorough cleaning it, then replacing the screw on element as well. Hopefully that alone would resolve the issue. If not, I would move on to the valve and inspect/replace the seals and verify the pump coupler is in good condition. If all those items are in proper condition, then the only option would be the clutch pack(s) themselves, and those will likely be the toughest to repair or replace.
As was pointed out, it may also be an issue with the dry clutch, operated by the clutch pedal. I'm certain these are available more or less off the shelf, and will be much more "normal" for a tractor mechanic to service.
