Question about upgrading 16.04 to 20.04

[Bret Busby]

Hello.

I do realise that, when the time comes, the upgrade would need to be
done in two steps; 16.04 -> 18.04, then 18.04 to 20.04. And, I do
realise that I now have about 11 months left before End Of Life of
16.04.

However, an issue has occurred to me, about which, I need particular
information about the upgrade process.

This all applies to UbuntuMATE.

On a computer that I have, I tried to install 19.10 (as the latest iso
DVD that I have), where a previous attempt to install 16.04 had
failed, because of MS Windows and it security boot stuff and UEFI. The
19.10 installation attempt crashed, and it had said that no UEFI boot
partition was found. So, I downloaded 20.04, wrote it to a USB
thumbdrive, and successfully installed that, without any problem. And.
20.04 installed automatically, as a UEFI installation, showing Windows
in the GRUB boot menu.

I have some computers that are running 19.10, so the upgrade to 20.04,
should be relatively simple and straightforward, hopefully, o those
systems.

But,in booting up another computer, that, from the system time of the
year 2015, appears to have not been booted for a while, but has as the
UEFI MS Windows, v8.1, and, booted automatically, as designed by MS,
into MS Windows. So, I went into the BIOS (/UEFI), turned off the
secure booting, and changed the booting from UEFI to Legacy, and,
booted it, to see what happened. It booted into UbuntuMATE 16.04, but,
the GRUB boot menu does not show an entry for MS Windows (in terms of
meaning that I can not boot into and use MS Windows from there, that
does not matter - I do not use MS Windows, and have not used it for a
number of years).

Now, the issue that occurred to me, regarding that computer (oh, and,
this one, upon which I am composing this message, and, my other main
16.04 system, both, in the GRUB menu, show the MS Windows bootloader
entry), and, applicable to my other 16.04 systems, arises from 20.04
having automatically installed as a UEFI OS, on the system mentioned
above, where previous attempts to install UbuntuMATE as a Legacy (as
in, via Legacy BIOS) OS, had failed; where, upon a system that has
UEFI and Legacy BIOS, and where Ubuntu 16.04 is installed as a Legacy
BIOS OS, with the booting and the GRUB stuff all in the BIOS part,
separated from the UEFI MS Windows bootloader, in the process of
upgrading, via 18.04, then 20.04, will the upgrading of the Ubuntu
version, also transform the handling of the OS, from Legacy BIOS, to
UEFI, so that the need to switch manually between UEFI and Legacy
BIOS, during the computer boot process, is eliminated, and so that a
single (GRUB) bootloader is used, that includes all operating systems
installed on the computer, with the upgrading transforming the
installation, into a UEFI installation, on each computer?

Or, would I need to do what I did with the failed installation
attempts into the (initial Legacy BIOS attempted) 16.04 installation;
installing 20.04 as a new installation, to replace what had existed?
If I would have to do that, then all settings and data would be lost,
on each system.

-- 
Bret Busby
Armadale
West Australia
..............