I've assumed that the cars Terry is referring to [ that MTH has not made in hi-rail ] are the 'second run' of LMS passenger cars, the Coronations and their subsequent liveries: hence, the following disclaimer: I do not have, nor have I tinkered with, these cars. However, I do have experience in converting other MTH Euro passenger cars [ Orient, OCEM, Rngld ] from 2R to 3R and vice versa, and have body shells from the first LMS run, so I may be able to contribute some information and suggestions, assuming MTH has used a common approach on the Coronation cars.
1. The concept of using the existing scale wheelsets on 3R track may work, as mentioned above, but there are a number of potential problems:
a. I've been advised by 3Rérs who have tried this that unless modifications to the turnouts are made, derailments, primarily when taking the diverging route, are likely.
b. You will not have any interior illumination from track power, because
- no center roller for pickup
- MTH trucks for 2R operation have plastic/nylon insert bushings for the axle end bearings, so that the all metal trucks are at neither + nor - polarity, and lighting pickup is by wipers on each side. The 3R version of the trucks have metal bearings, and hence both wheels, the axle, and the complete truck are at "-" [ = outside rail ] potential for lighting circuit return.
c. Again, as mentioned above: You will not have the truck mounted couplers found on 3R cars, which may be helpful depending on your minimum curve radii.
2. I would suggest substituting complete MTH 3R trucks for the 2R ones. MTH does use a bolt-from-top [ through a round metal 'bolster plate' screwed to the bottom of the carbody ] approach, going into a threaded boss on the top of the truck crossframe -- it's anything but the more conventional "hole in the center of the truck" approach.
Unfortunately to the best of my knowledge MTH does not sell replacement trucks, either 2R or 3R, for their British passenger cars -- and if they did, they would probably be $ 25 each, based on their other truck prices. Hence, I suggest the following:
A. Buy the cars you want with the scale wheels NOW, while they are available and discounted, even if the conversion solution is not immediately at hand. It is possible another vendor will pick up the MTH tooling [ since the tooling for the other MTH Euro passenger cars has been or is being* bought ] and produce them in 3R, but I wouldn't count on it.
B. Buy a suitable number of the MTH LMS passenger cars from the earlier run with the 3R trucks, and swap the trucks onto the Coronation cars; then sell the resultant 2R cars. This is the best approach, unless you value your time at 7 cents/hour or just prefer the challenge or headaches, or both There are other solutions of course, especially if you don't bother with car lighting, at least track powered -- I usually remove it.
I hope the above is helpful. Have fun.
[ *The potential buyer said on April 25 "We have temporarily stopped OCEM and CIWL tools purchasing process with MTH....[ because ] ...it is impossible to organize injection tests [ to check for rust, etc ] in the factory where they are located, because it is closed !" ]