Yes off anniversary changes can be done.
If the group renewed in 2017 and hasn’t renewed in 2018 yet, and they add a dual option the current plan would need to be ran with 2018 rates. Also they will need to change their anniversary. This is true if they just make a change regardless when the group renewed.
If it is 5 months from the anniversary date the group is locked out and you have to wait until anniversary.