If you've already flown the outbound, you can change the return for just cost of change fee. (As long as same fare booking class is available) It doesn't need to even be same routing, just one that is valid for your purchased fare rules.

If same fare class isn't available, then you'll have to pay the difference between your purchased fare and the next fare class available (based on fares from time of booking aka historical fares) OR if difference is less than change fee, the change fee.

If you simply buy a new ticket, itll be at current prices and will be a OW ex-EU thus incur carrier surcharges.

In your case the $300 fee is the cheapest option.

Note - to stress this is only doable when changing the return. It does not apply to making changes to outbound.

Theres no way to know value left if you decided to change the return to fly from somewhere else or much farther out (past max length of stay). Wont find out till you go to rebook that ticket. You may even find the ticket has a significant negative value in this case.