I found getting the body off a bit nerve racking. The instructions with the loco are rather non-specific to say the least. Trying to work out how much force to exert while terrified you are going to break something isn't really my idea of fun. Still, needed to be done as i want to get a decoder in.
The actually rewheeling is quite straightforward. The bottom keeper plate on the chassis pops off with a small screwdriver under each catch on one side. There is an arrow on the inside of the molding which appears to need to point towards the centre of the model. You won't notice which way around it goes as you take it apart as you don't know its there. However once you have reassembled the keeper plate the wrong way around you will wonder why the inside edge of the keeper plate isn't sitting flat.
There is a fair amount of clearance between the frames. They came off very easily on my model. I found I needed to remove the rib on the inside to get clearance for the P4 wheels.
Removing the one of the OO wheels was just a case of brute force, however taking the gear of the axle requires more care. A gear puller helped. Threading back on a new 2mm axle was much simpler, the only problem being making sure it was central with a set of calipers.
I purchased a set of Gibson Warship wheels, they are about .4mm under size but I'll live with that.
The wheels went in OK, but had to bend the wipers quite a bit to maintain pickup. Not helped by the fact the wiper is lined up with part of the wheel which is now plastic where previously it was metal.
Now a big problem seems to be the sand boxes. There is basically no way these will fit as designed. Looking at the drawings there seem to be two issues. Firstly the buffer beam seems to be mush too thick. The model is about 1mm thick and this needs to be thinned down quite a bit. This isn't easy to do as the pin for the buffers means it is had to get a smooth long stroke with the file.
You can see the 'Before' and 'After' view here. I've only taken the corner off to give some clearance.
Also the actual sandbox looked to be very much thicker than the drawing. Again I've taken quite a bit of that off and also take a bit of the end of the bogie side frame to bring it inboard a tou. The result is that you can just get enough clearance to get the loco around my 40" curve. Phew!