That would fundamentally break the decentralization model that Bitcoin is attempting to achieve. Yes it can be done, but it wouldn't be Bitcoin anymore. I still haven't figured out what PoS solves that centralized agencies don't solve better, but I guess that's a different topic.
Ye olde definitive criticism;
https://download.wpsoftware.net/bitcoin/pos.pdfAgree, PoW is such an important part of the network that changing to an PoS would means everybody is switching to an alt. I don't think you can achieve consensus on that because of all the stakeholders. Theoretically it is possible, there are some PoS networks.
