I'm new to Ubuntu and just finished creating a USB installation drive with Ubuntu 16.04.2 LTS iso file. I plan to rewrite my entire drive, using only Ubuntu. I'm ditching Windows for good. After completing the USB drive, I read about the different flavours of Ubuntu and would like to try Kubuntu. Do I need to create a new USB install drive using the Kubuntu iso file or will I be able to select different flavours once Ubuntu is installed with the USB I've already created.
I disagree with the advice given above.
You can easily install as many desktops as you wish. I personally use gnome, LXDE, openbox, and KDE depending on my mood and they are all insalled without problems.
In addition you can use kde apps in gnome and gnome apps in kde, etc . For example I prefer k3b and use it in all DE as my disk burning software of choice, no problems.
See What kinds of desktop environments and shells are available? for a detailed description of the most popular desktop environments and how to install them.
To install kubuntu
sudo apt install kubuntu-desktop
Installing multiple desktops will take up disk space, perhaps use a little more ram, and there will be a few duplicated applications such as text editors, terminals, etc and your menus will be a little more cluttered but they will not harm your system.
You then choose which desktop to use when you log in.
Removing desktops, however, is difficult and not recommended.
Results of removing can be mixed and there can be residual KDE apps and/or unwanted gnome apps removed.
Most people try different options and over time tend to use the one they like best and just install the appropriate version of Ubuntu they use.