If it's using the same bearing kit, then the shaft diameter must be the same - all that remains is the length. I checked the Haynes hardcopy - can't find any reference.
So the discs are .5mm thinner on the old H & J model. Question is also would the 87/88 H&J model calipers bolt directly to the 91 L model fork legs ? If you're going to attempt to bolt on a complete 91 front end, why not use the rotors and calipers from a 91 as well ( unless they are very expensive, or you don't have them) ?

Replacing the entire front end is not a mod I would do, as I don't think there's much to be gained from it - I'd be more inclined to rebuild the original forks, and maybe add a slightly heavier oil, or fit progressive springs to stiffen the front end a bit. Maybe dropping the forks around 4mm in the clamps (as suggested by Dean0 for a later model) may improve the front end performance ?
Also, as Honda Hurricane suggests you can jack the rear suspension up by moving the bolt hole on the lower suspension arm - moves the rear up by as much as 4".....again, if it's not done exactly right you could cause more problems than you solve.

 

When I started this thread, I originally only wanted to change wheels to get a better tire selections (got the wheels).

Then I thought why not change the fork "if" it would perform better than the current one, since I had access to one. If there would not be a performance gain, I "would" certainly stay with my current fork assembly.

Is it possible to change oil without removing the forks??

 

Yep, it's all in the manual......
I've not had to do mine, but I'm told it can be messy
if you don't follow the steps.[X(]