As has been pointed out, a Druidic focus and a holy symbol are distinct from an arcane focus, but given they all perform essentially the same function, and the same issues probably apply to all of them, I’m going to leave this question broad to apply to all foci.
At the core, I believe my questions are:
- is a focus any item of a suitable class (staff, wand, etc) which the caster is somehow “attuned to” (in the general sense, not the 5e mechanics sense), or is it a specific property of certain specific items (ie a staff must be specifically manufactured as a focus to be one)?
- if only certain items can be a focus, is there any mechanism to make an item into a focus (e.g. similar to how a weapon can be silvered)
Details for my specific issues:
One of my players (Druid) started at L1 with a Druidic focus (staff). Later they picked up a better staff (staff of bird calls), with some magic capabilities. Can they use the magic staff as their Druidic focus, or do they have to carry around their starting staff forever?
Similarly I have a sorcerer who’s picked up a staff of defence. Does he need to hold both staves to use the staff of defence’s capabilities and to cast magic using his arcane focus staff, or can he somehow designate / transfer the focus to the staff of defence?
I’m going to have a similar issue with a paladin player who has their Holy Symbol on their shield, as soon as they find a +1 shield. Will they have to choose between using the +1 shield for the AC bonus and their original shield to be able to cast? Or can they ‘swap’ the holy symbol onto the new shield? Would this require a blacksmith or a ceremony or something?