I called Apple Support to explain the grayed-out iPhoto situation. The advisor told me that this is a rare situation. Since my iMac was less than a week old, and was not yet backed up, the rep told me that based on my iMac's serial number, he could do an erase and re-install that would install the OS my iMac shipped with. I thought I would have a fully restored version of iPhoto. The re-install failed. A window opened that said "Can't install the additional components needed to install OS X." Another call to support, and the second rep told me the first rep was wrong, that once the system was upgraded to 10.10.3, only 10.10.3 could be reinstalled. I was resigned to the fact that I was stuck with 10.10.3. The re-install was attempted again, and this time it was successful. Since the reinstall, there is no iPhoto icon of any type anywhere, and I was told that iPhoto is no longer an option for me. In the meantime, I started playing around with Photos to see about making albums work like iPhoto's events. I can organize albums by date, and select key photos. Unlike events, the date of the album doesn't show until I open the album. In events, I could wipe across an event with my cursor, and it would show the photos in the event without opening the event. That was a useful feature, and I hope Apple includes this in future updates --and maybe there's another solution, but that's another topic. Keywords import from iPhoto to Photos, but only for jpg files, not RAW formatted originals.Not sure, but that may have more to do with the nature of RAW files than a problem with "Photos." Again, topics for another thread.
Apple was sorry for putting me through the gray iPhoto icon situation as well as misstating what could be accomplished with an erase and re-install. The last rep I spoke with said it was never Apple's intention to disable iPhoto on machines that had it when shipped and were just being upgraded to 10.10.3, but once the upgrade is complete and iPhoto is gone, there's no getting it back.
So I will just relax and live with it, see what improvements come with the next updates to "Photos." And yes Terence D, I found out how to store the original large files in Photos since I started customizing its features this morning.