Hi, can't answer this one fully but just give you my limited experience - namely, I flew MEL-MAN with the MEL-DXB leg in F and then the DXB leg in J.[NB: the F leg was an upgrade and the DXB-MAN service is a 2-class aircraft]. Whilst in transit at DXB for c3hrs I was still allowed to use the First Class Lounge, even though my ticket out of DXB was just business class.
Lumpster