Upvote:3
Price the return trip as a separate ticket, but before you buy, call Etihad and see if they'll give you a deal on a round trip.
Etihad is a first tier flag carrier, and they tend to price round trips quite differently than "two one-ways". And round trip tickets are as a rule more favorably priced (otherwise everyone would just book two one-ways lol).
You can game this out by "going through the motions" of buying all new tickets in the ticket booking engine, try it either way and see. But those are only hints... talking to Etihad will be definitive.
It's impossible to say in advance, due to the way demand pricing works.
Upvote:6
Some tickets can be converted, others not.
Pricing is impossible to predict. Price it out both ways and go with the version you like best (availability, price, flexibility, etc) .
If you keep it seperate tickets, you can also fly the return with a completely different airline, so there may be better options that Etihad.
Upvote:15
It is often possible to do radical alterations like this to tickets. Even in the worst case, where the ticket is completely inflexible, you can still cancel it to get your government taxes back and save a few dollars by applying the money to a brand new ticket.
Where it makes any financial sense is going to depend entirely on the ticket conditions: sometimes the change fees will be so large that you save nothing compared to just cancelling and rebooking. The good news is that one-way tickets on full-service carriers like Etihad tend to be comparatively expensive and flexible, so your odds are decent. It's usually not possible to do this kind of thing online, so call up the airline and ask.
Anecdote: due to a last-minute change in plans I once had to change an existing Etihad booking from a simple A to B return into a C-B, B-A reverse open jaw. This turned out to be cheaper than the original ticket, so Etihad gave me the itinerary I wanted and a partial refund on top!