RE:Key is WPMI'm no expert on WPM or their streaming portfolio, but according to their "streams" webpage on the wheatonpm.com website maybe they have a precedent for downsizing a streaming agreement. According to the streaming info they provide for the Yauliyacu mine,
"On March 23, 2006, Wheaton Precious Metals entered into a silver purchase agreement with Glencore to purchase up to 4.75 million ounces of silver produced per year for a period of 20 years, based on production from Yauliyacu’s mining operation in Peru, ........ On November 30, 2015, Wheaton Precious Metals amended the silver purchase agreement to extend the term to life of mine and effective January 1, 2016, Glencore will deliver to Wheaton Precious Metals a per annum equal to the first 1.5 million ounces of payable silver produced at the Yauliyacu mine and 50% of any excess. The price paid for each ounce of silver delivered under the agreement has been increased by an additional US$4.50 per ounce plus, if the market price of silver exceeds US$20 per ounce, 50% of the excess, to a maximum of US$40 per ounce".
I haven't looked at the historic silver delivered to WPM for this mine but the notational values seem to imply that the delivered silver has been decreased and the price Wheaton has paid increased. Considering how freakin sweet the San Dimas stream has been to WPM you would think they would be open to taking less and keeping their San Dimas gravy train rolling along.