I have a DHCPv6 daemon that is answering for requests of some machines and everything is working fine. So I decided to remove this DHCPv6 daemon e start another daemon on another server, but the problem now is that since all machines already have a long lease, they don't ask to renew the lease or to get a new lease. I know the solution would be restarting machines, restarting clients, using dhclient -r to release the lease, etc. But I can't do that, I only have the control of the new installed DHCPv6 server, is there any way to make DHCPv6 daemon to actively advertise to the clients something like that ? I'm using the ISC DHCP server.
No, you must perform a manual refresh/restart on the client or simply wait for the lease to be updated at 50% of the lease lifetime. This should not present a problem.
If you are concerned that the new server will issue conflicting adresses, see IP Address Conflict Prevention in the dhcpd.conf man page.