Can i disassociate the load balancer NLB1 from endpoint service and associate another load balancer NLB2, in order to migrate the users to another service(NLB2) ?

Yes, but with a short downtime.

1. Be sure to have NLB2 correctly configured, healthy and able to serve traffic.
2. Find your "Endpoint services"
   1. Visit the "Endpoint services" page
   2. select your endpoint service
3. reject connections (This will start the DOWNTIME period)
   1. open the "Endpoint connections" tab
   2. select the endpoint connection (the one coming from ACCOUNT2)
   3. click "Reject endpoint connection request"
4. change the load balancer
   1. visit the "Load Balancers" tab
   2. click "Associate or Disassociate load balancers"
   3. remove the association with NLB1 and add NLB2
   4. Save Changes
5. re-enable connections
   1. open the "Endpoint connections" tab
   2. select the endpoint connection
   3. click "Accept endpoint connection request"

The connection will enter a "pending" state. Once it will transition to "available" the traffic should be served again.