Conversion of a scope to a DHCP Only scope or to a BOOTP Only scope is not allowed when DHCP and BOOTP clients both exist ...

Conversion of a scope to a DHCP Only scope or to a BOOTP Only scope is not allowed when DHCP and BOOTP clients both exist in the scope. Manually delete either the DHCP or the BOOTP clients from the scope, as appropriate for the type of scope being created.