My understanding is that there are additional rules that apply to all fares, unless contravened by something in the specific fare rules used to ticket.

Be aware that when a ticket is changed, there seems to be some sort of notification to the plating airline. I purchased an ex-SEZ ticket and about a month later, since the fare was still there, had my TA change the dates and the route of the last flight. It auto-priced and I had new e-tickets right away. But the next day BA notified the agency that the ticket was invalid and ordered them to cancel it and refund it, withholding the 5% cancellation fee and the $125 change fee. My TA created a new itinerary and ticketed it, and I of course have not changed it. But the act of changing the first ticket, even though it auto-priced, caused it to come to BA's attention. I'm upset at being charged the 5% cancellation fee when BA was the one who ordered it cancelled, and the $125 change fee as well. But I am afraid of doing a charge-back with the credit card company because I don't want BA investigating and then cancelling the new ticket.

The fare rule seems fairly new to me... (I was worried that it was the rule 10+ years ago).

it allows 2 stopovers in US (after south america instead of one of them being transfer without stopover) and Asia (between europe to australia)

Yes, It's particularly significant when airlines join and leave OW, if one of those airlines is the only OneWorld carrier to serve a particular city.

I did not fully understand the implications last year that allowed for 2 stops in Asia instead of 1 stopover plus a transit between SWP and Europe. This is a very big change imo. And it also states that in the EF version of the fare rules (my copy is from Sept 2018)
The only problem reading the EF version of the rules is that 2/3 of the 2500 lines appear to be concerned about travel within the African continent and within specific Asian countries in the transfer section. I just plan to depart and return to SEZ and avoid the rest of Africa. I wonder if one can ignore the transfers section. Any of our ticketing experts (CALCHAS or JAXBA) care to comment?

I agree

This sounds like an extension of the standard mechanism used to choose the validating carrier.

Personally, and some here know that I'm pretty conservative on this issue, I see no problem. The airlines are now fully aware of what happened.

Are your tickets on ba stock?

Issuing airline is UL, 603..... although the 1st 4 flights are on BA. I intend to fly the 1st 3 BA flights, then redo the whole ticket. I guess that I will have to deal with UL, but may pay a fee for the issuing TA to do all of the work. 1st flight is next year. Anyone changed their ex-SEZ ticket after flying 1st segment? My copy of the rules from EF stated 2 entries into N America is OK, no need for 1 to be a transit w/o stopover.

There are data point that ba is very difficult to work with for ba issued ticket. I have ul and la tickets myself. Will see how it plays out.

I have found changes to a BA rtw ticket trivial, if you are not in a rush, ever since I found they have a rtw department you can email. It takes them a working day or two to assign someone to a query but thereafter responses are quick and phone calls are possible.

(Yes I will have the possibility of higher taxes to contend with of course but for ex cai as mine was or ex sez as it sounds you have there is limited choice)

That sounds good in general, but I am not sure it is a good idea to contact them for changes to an ex-SEZ trip, given the reports.

There were reports of a US RTW desk for BA at (800) 828-7797‬ but I have never tried it myself.

BA has an RTW dept in the UK [email protected] but as anabolism wrote there may be issues in trying to make changes to the ex-SEZ tickets.

That's the one I've used. I had similar concerns re my ex-CAI booking. In that case I waited till I had flown the first sector but I guess if BA decided it was never a valid fare rather than no longer a valid fare you may want to be extra cautious.

Clearly it was a valid fare, BA revisionism notwithstanding. That's what the SABRE guarantee is about. That said, I would have to assume that BA will be looking for any opportunity (excuse) to restrict the ex-SEZ tickets. As I mentioned above, I'm extremely (possibly even irrationally) conservative about this and thus will not be attempting any changes. IMO these tickets represent far too much of a saving for me even to consider taking any risk.