I'm pretty sure that under the current system, they have to provide them, but not necessarily straight away. Mercedes updated the 2015 engine with some parts for 2016, but chose to hold upgrade back from the customer teams because those parts were experimental and unproven, and they didn't want to compromise the customer teams with a potentially unreliable engine. The cynics among us will see it as an attempt to hamstring customer teams whilst staying within the letter of the law, but I think it's a valid point - the old-spec engine is a very good one, and while Mercedes themselves are dominating, some of their customers are in close fights with other teams for WCC position. The last thing anyone (least of all Mercedes) wants to happen is for a customer team to get an unproven upgrade, only to blow an engine, take a grid penalty, and lose position to their rivals in the standings.