When you try to switch the IP to Dedicated (Tools & Settings -> IP Addresses), the error occurs:
Cannot change the type of IP address already allocated to a user. IP address 184.108.40.206 is already allocated to user(s) as shared IP address.
Software issue PPPM-739, which will be fixed in future product releases.
As workaround use the following CLI utility:
%plesk_cli%\subscription.exe -u <sbscr_name> -mail-service-ip <new_ip>
To avoid such problems with other subscription, use this utility to change the IP address whole subscription:
%plesk_cli%\subscription.exe -u <sbscr_name> -ip <new_ip>