If you use Basic TeX, then I'm afraid the answer is Yes. Basic TeX is designed to install a minimal distribution and then all you to add any extra packages you need manually using either
tlmgr from the command line or the TeXLive Utility program that comes with MacTeX. These packages are added to the main distribution tree, (
/usr/local/texlive/<year>/) and are not installed in your local
texmf folder (
~/Library/texmf). Packages you install in your local
texmf folder do not need to be reinstalled, but packages that are installed using
tlmgr or TeXLive Utility do need to be reinstalled.
As mentioned in the comments, TeX Live is on a yearly update system for reasons outlined here:
So when you update a Basic TeX installation, you get a new minimal distribution for that year, just like you would get a new full distribution if you had installed the full version. Therefore any packages you added to that basic distribution will need to be reinstalled, for two reasons: first, the current year's installation has no way of "knowing" what's in another, earlier year's distribution. Second, and more importantly, packages from an earlier year may have been updated in the current year and if you used a previous year's packages you could well end up with package incompatibilities.
Unless you're really short on disk space, I would really recommend installing the full distribution, since this would save you the hassle of reinstalling missing packages manually.