AA would not give you a voucher in these cases.
They would convert your ticket to a Flight Credit where the ticket would be converted to open status and you could fly the return within validity to/from destinations combinable with your outbound fare and pay the change fee, and any applicable fare difference according to the historic fares.

So if we consider your example, you would not get a $850 voucher that you could use on any AA ticket but you could use it only something combinable with your original ticket.
As the poster above mentioned, it might be a good idea to look at the fare rules for each component and interpret them before you purchase the ticket.