I am in al safwa at the moment too, travelling on a revenue J ticket that I upgraded to F with qcredits several months ago.
I was also in here last week on a qmiles redemption F ticket that I booked the day before travel.
My hunch for what it's worth is that access depends on whether you are actually ticketed in F (or J for al mourjan), rather than just cabin shifted.
If you upgrade at the airport you don't get a reissued ticket. And that may also be the case if you upgrade in the day or two before travel once the flight is under airport control (someone may correct me on that). In both of my examples, I was issued with a new or revised ticket for the class I was travelling in, and had no problem with the corresponding lounge access.
That's just my theory mind, based on my experience and reading some of the tales on here. Given that the boarding pass scan ultimately says yay or nay, there must be some logic to it...