What i meant was it's possible to combine multiple consensus algorithm before the cryptocurrency is released, but it's not possible to add/remove consensus method on running network (at least without hard-fork)
But Vitalik is incorporating a new consensus algorithm right? So he's actually changing the consensus algorithm on a running network right?
Using a hard fork (as mentioned in ETFbitcoin's brackets).
So it's not impossible, but very hard and risky, depending on the size and activity of the network (both from a technical and a social point of view, ie. the larger the community the harder it is to consent on upgrades via hard fork).