Q: iphoto library upgrader

I would wait, before you uninstall iPhoto, at least until you are very sure, that each and every photo in your iPhoto Library migrated well. You may notice incorrectly migrated files only much later, when trying to edit  the photos, and it may be necessary to rebuild your iPhoto library in iPoto before you try to migrate again.

Also, Photos is only the first release of a very new application, and not all features, that iPhoto supported, are already supported in Photos. You may want to use iPhoto occasionally for things, that are not supported in Photos.  It will be difficult to reinstall iPhoto, if you find, thaat you would rather revert to iPhoto, since apple stopped selling iPhoto.

The migrated Photos library does nor need much extra space, since your iphoto Library and Photos library are sharing the image files. Deleting the iPhoto Library would not free much space, so you can keep it for a while.

See:    Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

When you are ready to burn your bridges and uninstall iPhoto, make a backup copy of your complete Mac.

Then you can delete the iPhoto application and  the iPhoto Library, also the iPhoto Library upgrader, but I would really wait a bit.

Personally I wouldn't delete any of them for a while. You won't save much space (complicated) if you do delete anything, but will burn your bridges for recovery if everything isn't as it should be.