Certain amendments can be executed without you needing to agree. By signing a purchasing agreement in the first place you are typically stuck to the contract especially if it is just dates being shuffled.
http://www.fic.gov.bc.ca/pdf/real_estate/FrequentlyAskedQuestions.pdf Page 6 in the Questions from Purchasers section.
Your recourse would be through the courts. If you're out a sizable amount of money as a result of their practices then you'd present that in a court and seek damages.
But quite frankly, amendments to dates happen in nearly every disclosure agreement and contract I've ever been witness too in large construction projects.
Again as I said earlier, lesson to you and everyone else to not put 100% faith and build your life and plans around completion dates until you're called in to do your inspection. At that stage you can be relatively certain the final completion date is more-or-less correct.
As nobody has gone for inspection on any units in this project, I would take even the current anticipated dates with a grain of salt. Yes it 🞵🞵🞵🞵🞵, but it is an unfortunate reality of buying off plans into a project not fully completed yet. I'd just suggest you contact Concord and try to speak with someone higher up the ladder about your concerns and annoyances. Wouldn't bother with E-mail it is likely being filtered by an admin assistant who isn't in a position to answer you at any length or with any substance.