CEP 2.1's data files are backwards-compatible with CEP 2.0 and NWN 1.68, and it does not include the top hak or tlk file from CEP 2.0 (both of which which contain index information specific to a given version of CEP and NWN), so you can install it over your current CEP 2.0 installation without breaking anything. You should also be able to install it over HG Enhanced without any issues.
When NWN 1.69 is released and HG upgrades, CEP 2.1 will be required to play Higher Ground. Although 1.69 is at least weeks off (if not a month or more), we recommend downloading and installing CEP 2.1 now so you have it ready for the update.
Of course, back up your current CEP 2.0 installation just in case there are issues. If you are running CEP 2.1 on HG (using the CEP 2.0 top hak and tlk file) and something seems broken that was not before, please report it. I personally have been running a beta version of CEP 2.1 for some time without problems.
When installing CEP 2.1, the files cep2_top_v1.hak and cep2_v1.tlk should not be overwritten. They should not be in the package, but this is just an additional warning.
Looks like the some of the files on the vault have been updated and fixed with the bugfixes I submitted. Download at will, but be sure to redownload the individual 7z files for cep2_top_v21 and cep2_core0, as at this time the large full packages have not had the fixes applied to these two files.
Sorry, that's what they said to d/l at the top, but you're correct. Judging by the comments, they mislabed the most current version. The 7z file should be CEP_2_Full_c.7z, updated 5/23, but if you want to play it safe and don't mind a longer download, you can d/l the exe instead, which is correctly labeled as 2.1: CEP21_Full_c.exe