1. Company A develops Project One as GPLv3
2. BigCo bus Company A
3a. usually here BigCo should continue to develop Project One as GPLv3, or stop working on it and the community would fork and it and continue working on it as GPLv3
3b. BigCo does a "clean-room" reimplementation of Project One and releases it under proprietary licence. Community can still fork the older version and work on it, but BigCo can continue to develop and sell their "original" version.
2. BigCo owns ProjectOne now
3a. Bigco is now free to release version N+1 as closed source only.
3b. Community can still fork the older version and work on it, but BigCo can continue to develop and sell their original version.
As a real world example, Redis was both Company A and BigCo. Project One is now ValKey.