You can transfer all your projects from one company account to another. This can be done as shown in the below demo. After selecting your projects, instead of exporting them, you click on the transfer button located at the top-right of the webpage (right next to the export projects button). Then in the next step, you can choose the company to which you wish to transfer the projects.
Please notice that projects transfer is only possible from one company account to another. Project import is not yet implemented, but it is in the development roadmap. Project import feature and also project sharing feature (sharing with colleagues within the same company) are planned for release in Q4 2021.