How to re install the system under legacy boot

I have managed my server through MAAS.IO (R720 / R730XD, BOIS is an old version boot, please forgive me for some reasons can not use EF), and have completed the deployment of Ubuntu 16.04.

Boot menu:

I now find that I cannot replace the server’s operating system with Ubuntu 20.04 through (Although the disk data can be manually deleted and then redeployed by, it cannot be automated).

I also thought about changing the boot menu to network as the first boot item, but after the modification, the OSD cannot be booted correctly after deployment.

Excuse me, is this step wrong?

Or does do not support lagcy boot?