Hello developer_rc,
This is not possible with PayPal Standard integration, because a Subscription Modification Button is designed to adjust an existing Subscription that is active already in PayPal; or to create an entirely new Subscription if the Customer is not yet paying you. Unfortunately, if the Customer IS already paying you, PayPal will ignore trial periods passed in with the Modification Button if at least one payment has already been received.
However, s2Member Pro ( integrated with Pro Forms ) does not suffer from this limitation; as it is still possible with s2Member Pro Forms to provide whatever periods you prefer, regardless of existing Subscription status. So the only solution I can offer at this time is to upgrade to s2Member Pro and integrate your site with s2Member's Pro Forms.
This thread may be of interest to you:
http://www.primothemes.com/forums/viewtopic.php?f=4&t=304