Its official:

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SUBJECT: Product Realignment - UoD using Qmiles/Qcredits
Process owner: Ground Services Page 1 of 1
According to the update received from Qatar Airways Privilege Club department, effective from the 06th of November, customers who purchase an upgrade upon departure (UoD) with Qmiles/Qcredits will be entitled to the baggage allowance and lounge access based on the actual ticketed cabin, and not the upgraded cabin.
However, frequent flyer members (i.e. platinum, gold, silver members) will continue to get their respective tier benefit when they purchase an UoD using Qmiles/Qcredits.
Example
A Gold tier member is booked on a revenue economy ticket, and purchases an upgrade
using Qmiles/Qcredits to the next higher cabin, the member will only get excess baggage and lounge access as per the gold tier entitlement i.e. :
 Excess baggage allowance of 15 KG
 Business class lounge access
ASMs to brief all concerned accordingly.
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