Well, after applying the one for my S6 edge in April, then the one for my Note 5 on September, and 2 more on December for 2 S6 I bought for my Wife and daugther, I indeed applied the 4 of them and still have $355 credit on my account on the UHD $11.99/month plan, still valid for 30 more months.
I know it makes sense that I can't do it, what I don't understand why I have been able to apply 4 of them before without issues and now I'm supposed to wait for the billing cycle to end. I have a bad feeling about it and think that I won't be able to apply any of them now.