In MOST omegaverse manga the Alpha doesn't change even if he formed a bond with his Omega....
TOWA is a UNMARKED omega so his heat and pheromones can attract any Alpha even if the Alpha already has a mate..
AIJI is already MARKED so whenever he goes into heat he will only attract SEIJI his mate...
SO ONLY the OMEGAs change...
But it is all up to the mangaka in the end...
Like in this manga's case:
http://www.mangago.zone/read-manga/pendulum_kemonohito_omegaverse/
Were if the Alpha beast bites an Omega they are mates for life and cannot attract others...
In Romantic Joutou, the Alpha wanted to get as many Omegas as he could, for his own reasons, but the Omega would get sick if he had sex with someone else other than him.
I Thought in the omegaverse if an alpha is already a mate, they can't sense other omega's pheromones?