We will not upgrade hashrate if we think we will not be able to deliver. A hashrate commitment is a hashrate commitment, whether that is for 1H/s or (after upgrade) 6H/s. Mining will start at Zcash launch, and you will have the hashrate we promised, but due to the experimental nature of Zcash there simply is no guarantee nothing can go wrong.
You could upgrade customers hashrate on launch day once you know exactly how much you are able to offer. Nobody asks you to make new, larger promises every week while keeping that clause that allows you to put all customers on hold for up to two weeks and miss out on the most profitable portion of the contract.