I know that HARRY RUBY WROTE the theme song, but
who sang it. Also, when the show moved from ABC to
CBS the theme song lyrics were revised because Kathy
Nolan left the series. Are both versions the same guy.
Who might he be?

Harry Ruby is credited for the words and music to all 6 seasons theme songs.
The first 5 seasons with Kathy Nolan don't give a credit to who sings it, but I always assumed that Harry Ruby was the singer, too.
In the 6th and final season, they got well-known pop and country singer Jimmie Rodgers to sing the revised open and closing themes. Unfortunately in syndication, the credit for Jimmie Rodgers was cut out. Perhaps a royalty dispute for paying him on the reruns. I've always wondered about this myself.

There's a lyric I have never been able to make out
in the original (season one) version of the opening theme.
What's is said immediately after the singer says, "and now
here's LUKE who beams with joy....(can't make out)..Kate McCoy.

And now here's Luke who beams with joy
Since he's made Kate Mrs. Luke McCoy.

Wow...thanks. That one has plagued me since my youth...
No matter how many times I've listened to it over and over,
it sounded like gibberish....until now.

Way back when TNN was showing the show, there was a clip at the end of the show in the credits that said Hy Averback sang the theme song. I can't remember what episode it was. I recorded many of them but in rewatching them, I still haven't found the episode. I may have missed that show in taping. It was definitely the early seasons with Kate though. If you've ever seen Hy Averback in any old shows, such as december Bride, the voice does sound similar, so it could very well be him.