Anybody has experience to use Avios to upgrade a segment from J to F ?

Ticket issued by RJ. Last segment in an RTW, LHR-MCT. What could happen?

You may have some better advice posting this in the BA forum.

You will need BA to re-issue the ticket on 125 stock, but that is not normally a problem for RTW products; though you may find that BA is a lot more exacting about YQ/YR calculations than other carriers are and wish to uplift the difference from you.

Other than that I see no objection in principle to an Avios upgrade of one BA-op, BA-coded sector on a BA ticket. But I would be interested to hear back from you.

You should ask BA directly, and also ask in the BA forum. I know AA and BA have published information that says that both Avios and AA miles can be used to upgrade flights operated by AA, BA, and IB and booked as AA, BA, or IB codes (so codesharing is fine as long as both the operating and selling carrier are either of the three), but BA flights need to be in specific booking inventories, which includes D. So, in theory, a BA segment on a DONEx should be upgradeable. However, I've tried to do so a few times, most recently several years ago, and each time was told that BA specifically prohibited upgrading an xONEx segment, although this does not appear on any published information I can find. So good luck, and please do let us know how it works out.

(Almost) all business BA segments can be upgraded using Avios, don’t need to be in D class.

There is a technical difficulty in upgrading non-prime sectors because the redemption selling class (into which upgrades are booked) are not loaded on codeshares.

QR has just re-issued/re-routed a LONE5 exCAI(now MCT) iniitally purchased Nov 2016. Fees within the range of "expected/normal". Seems to be willing to convert to DONE5 at reasonable cost. Waiting for my brother-in-law's LCIR29SA to be ticketed..... The weather confusion in the USA is apparently delaying AA's rate desk response.

Happy wandering.
Fred

Can AA work with a fare of another oneworld airline?

I don't believe so. In my experience with ex-CAI fares, AA is unable to reissue tickets unless AA had a published fare on the original issue date.

Although, if AA has a schedule change on their flights, if you are lucky enough to get a CK agent, the agent is able to do a limited reissue just covering the affected AA flights.

I've seen that with mistake fares. The airline will honor the ticket only exactly as issued, and any change at all will result in a reissue at the correct/current price.

It seems to be happening with HX fare LAX PVG.

It is reported that changes will revert back to economy class. I still have no idea why it was booked into J class at all.

I have a QR-plated agency-issued ex-CAI DONE5 that starts in early February. I want to make date changes to the last few flights. The agency says they cannot do the date change until I fly the first flight in February. My concern is that one of the flights I want to change (AA) has gone down to D2 on the date I need to move to (two passengers, so I need D2). I tried to get AA to upgrade a flight and re-issue, but AA said they couldn't because the ticket shouldn't have been issued in the first place, as there had been a memo to all OW carriers to pull ex-CAI fares. The agency says I can call QR to make the change if I want, but I wanted to check here before I try that; I obviously don't want to risk the ticket.

See if your agency is willing to try to book the new date without cancelling the original segment (i. e., double booking) for you.

No go:

book up all the seats using refundable tickets?

This doesn't mean that when you cancel the booking, the seats will go back to the D inventory.