As soon as I wrote the subject, the forum system pointed me to Want to change references to parts that I've already laid out, which answered my question.
However, this doesn’t make much sense to me — on the one hand, why would the default be “by reference”? (when reference is something that can very easily change, including after the part has been carefully placed/laid-out).
But perhaps my most fundamental issue is: why would it be up to the user to deal with something as abstract and mysterious as a “timestamp”? (I know, users of this software are likely engineers … but not necessarily software engineers or engineers with any experience in software development).
Why wouldn’t a footprint be identified by which footprint it is, instead of being identified by its label (which, ehem, can change!) or by some mysterious thing called “timestamp”? Why not assign an internal id acting as “primary key”?
BTW, if the timestamp is the timestamp at creation and is unique (in particular this second part, I’m not sure), then why bother asking the user — just always identify the footprint by its timestamp (am I missing some valid use cases where the timestamp would change?)