The move to PoS will of course not be done by miners, by definition.  It will also obviously be a hard fork.  There's no way to implement PoS by a soft fork.  So the obvious way for PoS bitcoin to emerge, is to do a hard fork.  Those wanting to remain on the PoW coin will do so (and the miners will of course be part of it), and sell their PoS coin version.  Those wanting to do PoS will sell their PoW version.

The only question that remains is simply: which prong of the fork will be entitled to the brand name "bitcoin".  This will be an exchange's decision.  If exchanges decide to call the newly implemented features "bitcoin", then bitcoin will "have switched to PoS" and the original one will now be labeled an "alt coin" (bitcoinhash or something).  If exchanges decide to keep the original bitcoin line called bitcoin, then the PoS version will be an "alt coin (bitcoinstake or something).  Most probably, the monopoly to the name "bitcoin" will be decided by the few people that have pushing rights on the bitcoin core centralized Github archive.  That's how the original bitcoin now became an alt coin, called, bitcoin cash, and the new version with other segwit technology, remained bitcoin.

As the market is quite technology-ignorant, and very brand-name sensitive, this will be an oligarchic decision between 10 or 20 deciding entities.