Changing the location or function of a control on a user is normally something that is frowned upon, and with good reason. But take this situation as an example:
Users need to specify a number items from a list, which they want to associate with a thingy. Some users may have only a few items, while others may have dozens or hundreds.
- If there is only one item, the association could be made for the user, with no controls given.
- If there are 2-5, perhaps a set of check boxes would be sufficient or appropriate to choose a subset.
- But if the user has 150 items, it might make more sense to use a "pillbox" or "tag input" box with autocomplete for entries; or perhaps a two panel "multi-select then add" UI would be more efficient, or maybe just a long table-format list with check boxes, or a list of checkable items in a scrolling window with a "Select All" option, or some crazy multi-select drag and drop doodad.
I'm not sure which of those would work best, the question is this: In the same UI is it ever ok to present ONE control to one user, and a DIFFERENT control to another user, maybe one with a different set of items? Would users get freaked out if they crossed the threshold and suddenly had a different control, or would it likely just make sense to them?