You can change your license, but it's not retroactive, the code released so far keeps the old license (and can be forked from there). Note that it would be unsustainable otherwise, e.g. release as free, force people to pay later
Isn't going forward sufficient?
If the code is already fairly mature then the big companies using it can just fork it (and make their new fork entirely closed if they feel like it) and the creator of the original code won't be able to do a thing about it.