Which system version do you currently have installed?
It will depend on your system version, how you can access the photos in iPhoto Libraries, but you can still access them.
iPhoto has been replaced by the Photos.app in 2015. The last compatibility update for iPhoto had been released for OS X 10.10.3 Yosemite. At that time iPhoto has been removed from the AppStore. But Apple kept iPhoto supported for several years for long time users to give them time for the migration to Photos.
The more recent versions of Mac OS have been designed for the new Apple silicon hardware and are best used with a new Mac. We can install the new system on some older Mac models, but you should not expect all older applications to run on a new system version. It is not compulsory to upgrade to new major releases of a system. Only security updates should be installed asap.
Otherwise, check before you install a system upgrade, if there are updates for the applications you need to use. if important software cannot be updated for the new system version, stick with your current system, if it is running well. When you purchase software, you buy it for a certain Mac model and system version. The system version is the foundation for the software to be able to run. Installing a new system version is removing the foundations your applications are built on. Don't do it, if you are not sure, if the new software will support your older apps. At least never install a system upgrade without making a full backup, so you can downgrade to the previous system version, if need be.
The final end for iPhoto came with macOS 10.15 Catalina, five years after the release of its successor Photos. Catalina has been the first only 64-bit system version where Apple removed all support for old application that still included 32-bit code. And that made iPhoto and iPhoto Library upgrade obsolete.
You will need iPhoto Library upgrader only for iPhoto Libraries, that are still version iPhoto 7.x.x. Do you really still have iPhoto Libraries that have never been opened in iPhoto '11 or iPhoto '9? I
To prepare very old iPhoto Libraries from the time of Power PCs, you would need to run iPhoto Library upgrader, and for that you would have to revert your Mac to an older system version - go back to macOS 10.14 Mojave or earlier.
But if you have already been running iPhoto '11, version 9.6.1 or so or even iPhoto '9, version 8.x.x you do not need to downgrade. On macOS 10.10.3 to macOS 12 you can open iPhoto Libraries in Photos for Mac.
Just launch Photos fro Mac while holding down the options key and select your iPhoto Library to open in Photos.
On macOS 13 Ventura this is no longer possible, but you can import the iPhoto Libraries into your Photos Library to access your iPhoto photos.
