Increase number of addtl members on sub without extending the main sub expiry

I've set it up a plan using the Family Sharing option so there's a main subscriber and X additional subs.

When I go to increase the number of additional subs in the subscription page it also increases the primary sub expiry date by adding on a year.

Can I just add and pay for further additional subs without it affecting the primary sub?

Thanks
 

So you've it based off CBSubs Options? No there's no way to increase the amount without doing a renewal. There are plans to accept a partial payment in a future release to allow buying additional options mid-subscription though. One way you can workaround this is have 1 plan for the family plan and 1 plan (e.g. merchandise plan) to purchase more family members otherwise they'll just have to increase the amount with a renewal.