I will have the exact same situation on 13. of December. When I click the "buy lounge access" button it shows access to first class lounge, business class lounge (both that one for Gold and Platinum members) and to AL Mourjan. Al Safwa is not mentioned. But funny, the luggage allowance is 50kg, so that one for First.....
Let´s see the outcome.

Good on you QR,

Compared to EY with whom upgrading to J with miles was a piece op cake I was much pissed-off this year by TRYING to upgrade all 4 segments of my last Y booking, remember you now need to buy your ticket in an upgradable (more expensive) booking class to start with (and that on itself is confusing)
I managed to upgrade 3 segments at airport with QCredits and miles, loosing a segment upgrade (airports upgrades with Qcredits can only be done for 1 segment, thus leaving me with 10 unusable QC's)

I decided NOT to book any Y tickets any more, NOT to care for miles accrual any more with any airline (they can stick them where the sun doesn't shine)
Yes my next first 2018 J trip is booked with QR because it was the best deal 6 months in advance - 2 seats BKK-BRU at 140K baht (even missed a better deal with QR 24h later...that QR20YEARS promo code, and yes I knew BKK-AMS came cheaper)

Thus I am not sure buying a ticket 6 months in advance is the best thing to do.
I fly BKK-BRU 2x year, will probably book J with any decent airline (QR, EY, AY, KLM, EK) when the price is less than 2k € and forget whatever advantage FF programs give, I think the only advantage flying J is the extra baggage allowance, as for miles accumulated when you can't use them...buy some duty free stuff???, or use them for extra baggage.
And I certainly will consider AMS, CDG, DUS, FRA as alternative airports.

Same for me, we will see.
B.

For those who worry about Al Safwa access with an upgraded ticket I can tell you that I'm actually in Al Safwa in the middle of a BKK-DOH-CDG in F with both legs upgraded from J with Qcredits months ago.
I just showed my DOH-CDG boarding pass and was given access promptly and even was offered a bedroom before I asked...

I upgraded at DOH today and was told of the new lounge policy (with regret expressed by the very nice agent) and I was no longer able to go over to Al Mourjan. I asked whether this new policy is just for airport upgrades and the agent confirmed that for now it is. However, he did then say it is going to be rolled out to all upgrades in future. Let's hope he's misinformed :(

As an aside, for those with no oneworld status who pay to upgrade on departure, wouldn't it be against oneworld rules for no lounge access to be given...?

purple1addict Oneworld rules regarding lounge access are, effectively, only valid for OWR, OWS and OWE passengers. And they also appear to permit that original ticketed class may be used.
What QR is doing is to eliminate the automatic upgrade of lounge access for airport upgrades and refer affected passengers to the OW rules for lounge access.

Here's a link to the OW lounge access rules. I didn't see any reference to original or upgraded ticket class.

Moreover, the second section is titled "oneworld lounge access for First Class or Business Class customers", which applies by reason of cabin of travel and not FFP tier.

all well and a nice read, but if the entrance dragon says NO, its no soup for you....

nilsfr69 at DOH you can always avoid the gate dragon by taking the lift up and talking to the much more knowledgeable receptionist!

The gate dragon stands at the bottom of the escalator, so I guess you mean to take the elevator up to the reception instead? ;)

Isn't lift (British) = elevator (American)?

Yes and yes. :)

Only a paid Al Mourjan pre departure access is showing as available for me in Manage my Booking. This is ahead of my U miles upgraded segment, DOH/MEL, mid Feb 2018. Original R/T booking class Hobart (HBA) Sohar (OHS) is in H. This suggests exclusions ( of AM lounge access in DOH) are being programmed into miles upgraded segments, already. Irrespective of "when" upgraded - and not only at airport- immediately before departure.

now if only someone could test the same with AS Lounge.