Hi,
I'll start with a
disclaimer:
I've never tried that kind of mix-matching in practice, so take my answer with a grain of salt.
Answer:
If cassettes have largest sprockets placed on a single spider carrying them, then it would be practically impossible to disassemble them and swap any spacers.
If the sprockets are all "full size" (each sprocket reaching all the way to the freehub, directly), then you could just swap the largest sprocket (if the sprocket thickness is close enough, you could also swap more than one sprocket from an 11-speed cassette, while using the spacers of the 12-speed cassette).
A picture might help explain what I mean:
Depending on how the cassettes you source are "stacked," you could use the largest 3 sprockets from the 11-speed cassette (if the largest 3 are mounted on one spider, as shown in the picture above), and use the 12-speed cassette spacers and other sprockets. That might also just work. Perhaps not ideally, but it might (in that case, the largest 3 sprockets would be a bit more widely spaced).
Things to consider:
Noting this just to be on the safe side. Make sure your shifter and derailleur model can work with a 12-speed system.
If you use the largest 3 11-speed sprockets on a spider, you might have problems mounting the whole cassette on the freehub if it ends up being a tad too wide (I expect the difference to be negligible, if using 3 or fewer 11-speed sprockets, but I haven't done it so can't be 100% certain).
Note:
There is little to be gained in terms of gear "tightness" when going from 11-speeds to 12. Even the difference when swapping from 7 to 8, or even 9 is not really that noticeable, unless you need some particular gear ratio. But 11 to 12 gives very diminishing returns. It could be an interesting project though.
Hope I've explained this well-enough and that it helps.
P.S.
My
cassette compatibility chart lacks the Shimano 12-speed cassette info - still haven't "played" with them.
Relja
Improviser Novović