How do/should we digest an area change ICly?

E.g., Arboria is no longer Arboria, now it's Eil whatever. In an IC sense, has it always been Eil whatever? What does my character that was alive before and after the change know?

With some things the answer seems apparent in game, i.e. Ashes/Forest of Nowhere. But for replacements, is there accepted canon? Would my character know the sirene lair had changed?

Live with it.

I've long ago stopped referring to the past as the past since it is such prevalent knowledge I refer to it in present terms.

I would say if the new area is supposed to have continuity with the old--and Arboria to Eil Shaeria is a good example of that--then IC you could handle it as either a change in names (places do change names over time) or as wider appropriation of an indigenous name (e.g., if we were to start referring to Florence, Italy as Firenza).

Other places that have merely gone away (without replacement) may have been lost or destroyed over time.