Transferring Business Premium Licenses Between Partners

ClaudioStallone unfortunately this would not be possible, as the 300 seats max is enforced in Partner Center on tenant level (for both Legacy and NCE). If the customer/tenant already has max amount of seats, another CSP partner can't provide that license to the customer anymore. 

And indeed there's no subscription transfer option for seat-based subscriptions in NCE. So you need to do it manually. I can only think of these 3 options: 

1. As the current partner provides 290 seats, you could do the transfer in steps of 10... might just take some time 

2. Or ask the current partner to first suspend their subscription and you provision a new one as soon as possible. But of course this will impact software availability of the customer, which is far from ideal.

Risk here is that Partner Center might need some time to sync, so it might take a long time before you're able to provision the new subscription, all the while the customer not having access to the software. 

3. Last option I'm thinking of is to purchase an E3 subscription, assign E3 license to all the users, suspend the current BP subscription from old partner, repurchase the BP licenses on your end, reassign BP to all the users, and then cancel the E3 licenses. 
If you do that in a single day, you should not receive any costs for the E3 licenses as you immediately cancel the licenses again.