I had an informative little chat with Matt. This was what he had to say:
Keycaps can be removed by simply pulling them off. On most keyboards I've dealt with, there is very little danger of damaging the keycap when doing that. But, I have a reasonably strong impression that the keycaps on the Lenovo 300 laptop are significantly more delicate than most others. The little plastic tabs on the underside of the Lenovo keyscaps that hold the keycap onto its support bracket break really easily. So, if you were intending to replace them sometime, but broke some while removing them, later replacement wouldn't be possible. Also, if the caps you want to use instead of the originals also have delicate attachment tabs. they might break when you try to install them. Have you thought of keeping the original keys, and writing up a set of stickers to put over the original keys - you could write whatever symbols you wanted on those stickers.
The fragility of the key mechanism [when removing a keycap] is enough discouragement for me---this is no IBM Model M keyboard. (The day they build a buckling-spring keyboard into a Thinkpad, I'll be all
over that.) The Lenovo line still has a lot to recommend it, however, and perhaps a ready-made set of key-stickers
is a reasonable way of "rearranging" the keys. I've had bad experiences with stickers on top of keys (peeling, tackiness, etc.) but hope that these are of a better cut.
Thanks, everyone, for your help.