Allow the 12-month subscription to add 12 months to an existing 6-month?

We have a six-month subscription.
Want to offer users a 12-month subscription too ( discounted a little on the cost of two 6-months)
I set this up and its fine for new subscribers but doesnt work as we like for upgrades,

If i take the pro-rated option off it starts right away with 12 months from today (just ignores the time actually left on the 6-month)
But if I switch that off it doesnt upgrade at all, it offers a pro-rated amount but then doesnt actually buy anything, its just marked pending.
I'm assuming its waiting to start when the old one expires?

What we would like to do is have it upgrade immediately  starting the new 12-month cycle ADDING 12 months to the previous expiry date (and closing off the previous subscription)

is this possible somehow?
 

Those are 2 different plans. 2 different subscriptions. You can't just add more time to it doing that. Prorating takes the remaining time on their subscription and turns it into a price value. That value is then discounted from their upgrade. So if they subscribed for 6 months and only used 2 months of their subscription their remaining value is 4 months which would then discount their 12 month upgrade.

To upgrade just the duration of a plan you could use CBSubs Options instead. This would let them change the duration of their plan and would just add more time onto their subscription when they renewed. To do this install and publish CBSubs Options then edit your plan and under Integrations > Options > Prices you can enable additional pricing options for a plan. Then during renewal they can change their duration from 6 months to 12 months.