Managed to ease the existing wheels out to EM gauge during the week. The bogie sides just ease outwards and they can be taken off, together with the pickups. The wheels are wide enough to utilise the original axles, which are splined. I drifted a hint of cyanoacrylate adhesive into the axle holes on the outside of the wheels, just in case. The pickups need to be bent outwards to suit the wider back-to-back.
The wide wheels look as though they are in sliding contact with the insides of the bogie sideframes. There are a couple of options here:
Firstly, one might be tempted to replace the wheels with narrower ones on longer axles, thereby clearing the sideframes. The wheels on a class 16 look to be a tad meatier than a wagon wheel, which is the only 14mm diameter 8-spoke to be had at the moment.
Secondly, one might be tempted to keep the original wheels and axles and insert 10-thou plastic spacer pieces, one each between the bogie end pieces each side and the bogie side frames, in order to increase clearances with a bogie that is now 20 thou wider than it started.
Time will tell the right way to go.
Ran the thing for the first time on the Surrey Arms layout, where it impressed with its smoothness and its quite limited top speed. Might give it a thrash with some wagons on the Test Track at ExpoEM!