User blog comment:Stbk22/Possible power up/Techniques 2 for New Arc/@comment-1884090-20160807150808/@comment-24.113.252.213-20160807180833

In the Rias vs Sairaorg Rating Game in vol. 10, Regulus did promote to Queen. He did it at the same time as Issei did, during the final match. Issei promoted just before he faught Sairaorg, and Regulus promoted just before he faught Rias.

So when Sairaorg fused with Regulus to use [Regulus Rey Leather Rex] against Issei, Regulus was already promoted to Queen.