Where exactly did this information come from? And why the five year gap between Pilot and Series?

Where did you read this at?

IMDB.

It would be interesting to find out if it's true or not. If so, then I would imagine there were some different actors involved in the pilot.

I doubt it's true.

I wonder if Kasey, an Angie fan, might know the answer to this?

It has to be a typo, unless there was an earlier attempt with a different actress in a pilot. Donna Pescow's role in Saturday Night Fever (1976) led her to being cast in Angie.

Rcbrad is correct, the information is false. Donna Pescow was cast after the success of SNF and back in 1974, Miller Milkis had just started HAPPY DAYS. Besides, anyone watching it can tell by the hairdos and look of the show it was 1979.