This is not mandatory, you can just use nvOC upgrade^* on your existing 2.1 install, and versions number will get bumped to 3.0. Re-running firstboot scripts, either manually or by doing a reimage will have the very same effect. Any image released so far for nvoc 2.1+  by default install whatever is in the current release branch. If you have already downloaded in the past one of this images and it installed 2.1 that same image will now install 3.0 if you do a reimage.

3.0 is the finalization of the 2.1 dev cycle. While 2.1 was getting major updates during development, now, as 3.0, this will only get bugfixes, miners and maintenance patches. That's why I would not suggest you to keep both 2.1 and 3.0 side-by-side. Instead you may want to keep both 3.0-stable and 3.1-dev. This last one will get new features when ready.




* if you do nvOC upgrade from 2.1 to 3.0, the miners submodule is currently left behind at 2.1: a fix for this is ready, you won't need to do anything to unlock this situation, just continue doing upgrades normally as usual