I’m currently designing a webpage where users are asked to modify the positions of certain elements. Currently I have a single button to allow the users to enter “edit mode” which allows them to drag and move positions of sticky notes on the page. This is the only form of editing users can perform on the page. Currently, upon hitting the edit button, it transforms into the save button, which users are expected to hit after modifying the elements. I’m currently wondering if I should have a separate save button, even though it has no function except for when in edit mode. Having separate buttons would also conversely mean that the move button then becomes non-functional when in the edit mode. I’m curious if anyone has any thoughts about having one conservative/single button versus two buttons which are only functional if the other one has been pressed?
It depends how obvious it is that the button has changed from 'Edit' to 'Save'. Both are four letter words so the button size / length would be pretty similar. I would suggest changing the button colour to make the transformation more obvious and to make the 'Save' action stand out. Also button position should be considered in relation to what's being edited.
The alternative would be to have a separate 'Save' button, but you say that would have no function except for when in edit mode... it sounds like you're saying both buttons should be visible all of the time in this scenario? I would only show 'Save' when in edit mode i.e. don't show 'Save' when not editing, so this still leads to a UI that displays one button at a time (unless I've misunderstood your description).
You should also include a way to cancel the edit so that it's really clear what's happening.
It seems you're concerned with double-purposing a single button on your page. This can be a major annoyance in my opinion when done wrong.
In your case, I don't necessarily think of it as a double purpose (especially if you're shooting for a minimalist UI). I think of it as a toggle button, and allocating that screen real-estate for the toggle button (edit on, edit off) sounds natural to me.
Side question: do you have a way to cancel without saving? It might seem a little less natural if you inject a second button on the UI and would kind of undo that feeling of "toggling" modes.