Will upgrading Desktop version to server need re-install of apps?

There is no "upgrade" to be done for that. It is somewhat the other way round.

If you install Ubuntu server and then want to convert in into an Ubuntu desktop, you just have to install the package "ubuntu-desktop". This will add everything needed for a desktop system (windowing environment, basic desktop applications etc.).

If you want to convert an Ubuntu-desktop system to an Ubuntu server, you would have to remove everything that you do not need any more.

If you install the meta-package ubuntu-server and uninstall the meta-package ubuntu-desktop and then do an "apt autoremove", this should uninstall all desktop applications, leaving only the server packages installed. (I have never done this myself, no warranty given that this produces the desired results.)

I cannot judge whether it is worth trying that.

The only difference between the server and the desktop version is that the desktop version has a GUI. The server is not an "upgrade" of the desktop version of Ubuntu.

If you want a MythTV server installing on your desktop OS then this is fine and makes absolutely no difference. The desktop stuff (Display manager and so on) just use resources. The server version of Ubuntu doesn't come with a GUI to give more resources to the services it provides.

It seems like I would be wise to clone the drive b4 trying?

Thanks for that everyone. My question is well answered now. I know exactly what's involved, what can and can't be done, and a safe way to try. Thanks again.
Dave