texlive, but I changed my mind and now I would like to install
texlive-full. I'm wondering if I can just install
texlive-full now, and it will just skip over everything that's already installed and fill in the rest, or if that will mean I two separate versions installed,
texlive-full. If it's the latter, I think it would probably be best to just start over with a clean slate, so that I don't have a bunch of unnecessary junk cluttering up my computer.
Both texlive and texlive-full1 are meta-packages, which means that they don't have any files of their own and consist only of dependencies, i.e., these packages basically install the dependent packages and nothing else.
On the package details pages you can see that
texlive has three packages,
texlive-latex-recommended. All three packages are also included in
texlive-full, with many others. Therefore, when installing first
texlive and then
texlive-full, then the dependency checking for
texlive-full will skip those three packages (because they are already installed) and install the others.
1The links are for the current Ubuntu LTS (20.04). This distribution contains TeX Live 2019, so two years behind the latest release (TL 2021). For normal use this is generally fine, however you might come across a new package or an answer here on TeX.SE that uses functionality which is not available in TL2019. On the other hand you will not have to deal with newly introduced errors, backwards/cross-package compatibility issues, and the need for frequent updates that vanilla users encounter (and at the point that Ubuntu starts packaging TL2021 these issues are likely to be mostly ironed out).