The way the multilateral convention works is that once you have opted into provisions, you cannot then change your mind and withdraw from them. The other impact is that the government would always have the choice to terminate the MLI, and would stop on a go-forward basis any amendments being made to our tax treaties. However, the effect of the MLI on the tax treaties that had already had a match and been covered would continue in the future. The government and another state are always free in the future to enter into bilateral negotiations to make any updates to the bilateral relationship that the parties desire.
On February 5th, 2019. See this statement in context.