My understanding of the Undercarriage bay is that it's just the bay area that is one Frame larger meaning enlarging the bay and not adding one additional Frame in this area. So really that shouldn't be too hard. This is different to the A340 -500/-600 Centre Section which added Frames to accommodate the -500/-600 double centre Boogie instead of the -200/-300 single centre Boogie.
The Wing being moved forward by one Frame I can only think would make the Trailing Edge in the same location given the 14 percent (? forgot where I read that) increase in Wing Area from the Trailing Edge extension (the latter of which should be relatively easy but means sacrificing the second kit's wings to make the bigger trailing edge to put on the extended kit but also meaning no left over wings to put on an A330 kit to make an A330 Neo or whatever it is called). The way the kit Wing to Fuselage joint is would make this easy and hard. Easy to cut out a notch on the leading edge and cut a slot off the front of the Wing location tab and then glueing the cut off slot at the back of the tab but putting this modified Wing into the very deep Fuselage "glove" would require a lot of filling and sanding as the "glove" is perfectly matched to the Wing cross section so moving the Wing forward in this "glove" will result in gaps.
Anyway with -900s being more prolific than -1000s and at the rate that Operators are cancelling -1000s and changing them to -900s as there seems to be stories everywhere of this occurring it's probably better off saving -900 kits rather than cutting two up to make a -1000 no matter how cheap the -900 kits were!
