Depending on your tightest radius curves, this is not a particularly difficult thing to do, but will not work on settrack curves of any sort. The chassis casting on the swinging truck versions is slightly different from the newer version with the fixed truck. The way I have done this is to, firstly, modify the trailing truck by cutting away all the metal outside the wheels using a razor saw as in this picture -
Then, taking your nice new X9834 spare part, get to work with the saw again and remove the portion containing the mounting hole to the chassis (the new style chassis is different in depth so you can't use the part, as supplied, on the older chassis. Cut the part in half by cutting the bar along the back so you now have two separate sides of the truck. Remove the excess plastic at the front of the frames just to follow the curved outline. See this pic, I hope this description is clear.
To fix the new frames you need to remove the pipe detail from under the cab and also the moulded "pips" etc. that these were mounted in. The replacement frames are glued in place,I used 5 minute epoxy.
From this photo,
you can see how the mounting screw hole has been modified to carry the tender draw bar - this has also the effect of closer coupling of the tender. The screw is a standard 2mm, I used a spare 3link coupling spring.
The pieces of plasticard glued to the sides of the chassis are to represent the firebox sides, this is to hide the "see through" effect of using the new frames.
These mods. will allow the loco to run on minimum 2 foot radius curves.
Hope this all helps.
Note that this conversion is not appropriate for "Princess" models as the spare is for a Duchess and the frames on a Princess are quite different ( No holes and fewer rivets!)