Renew membership length

Hi, 

i have a plan of 1 year subscription. 
i want to extend time of a plan by 6 month as a one time gift for anyone holding subscription today. 

i did it before by changing the period of the plan, and then renewing subscription for all the users. 
it seems something has change, and when i'm renewing they get a new subscription with one year (the previous subscription time)

any idea how can i do that?

thank you

Hello

Maybe it is because your plan has fixed expiration date? Mean all subscriptions expired on the same date?

Tuan