Bill, if you're doing an MTH enigne with the 3/2 rail option (all newer engines) it is a breeze- you can swap wheelsets out very easily by taking out the 4 small screws on each truck block, lifting off the cover and removing the axles/wheelsets. Next you drop in the replacements and put the cover back on- as easy as that.
Not sure about other manufacturers' engines as they are not made to do it quite as easy--