[smcphile]

No, open source licenses impose no obligations on the copyright holder for their own code. For example, if the copyright holder releases their own code under a GPL licence, no law prevents them from also releasing a modified or unmodified version under a proprietary licence. This is allowed by law, but the proprietary version doesn’t of course negate the rights already given to those who received the GPL version.

If a project has many contributors (copyright holders), they would all have to agree to a license change, so in practice something like the Linux kernel is very unlikely to ever change to a proprietary licence, but it’s theoretically possible.

The copyright holders are never bound by the terms of a user licence.