Booting new device from external ssd with older MacOS

Hi, I would appreciate your assistance.

I’ve been using an external SSD (SanDisk Pro) to boot my late 2013 iMac, and it has been working great, giving my iMac a new lease on life. I’ve relied on this SSD for several years, and all my data is stored on it. However, I’m now ready to upgrade my device and purchase a new M3 MacBook Pro.

I have a few questions:

1. I would like to continue using my SSD on the new M3 MacBook Pro. Will I be able to boot from the SSD and use it as I did with my iMac? The SSD is currently running macOS Catalina.
2. Will I be able to upgrade the software on my SSD to be compatible with the new M3 MacBook Pro’s operating system? If so, how safe is this process, and what is the best way to go about it?

Thank you in advance for your help.