The copyright owner of a work determines what license to use when distributing their own property (code in this case); so the company can decide to switch to a more restrictive license at any time if they so wish.
Note that that does mean they must own all of the work, or have obtained licenses for the code from whomever does own it that grants them the right to re-license the work under a restrictive license.
However, code previously licensed under a permissive license, gives others the right to distribute that code to anyone they wish to. So, old code, once distributed under a permissive license, can be re-distributed under that same license. In theory that means the old code remains available to everyone as Open Source software, provided someone continues to distribute it.
The owner can decide to stop distributing the code under the permissive license, but as the older versions have already been distributed with a permissive license granting the right to re-distribute without restrictions, they cannot prevent others from doing so.