"The Partridge Family Movie"

I was watching "Come On,Get Happy:The Partridge Family Story",and I was wondering about some things on the TV movie:

1. How come the theme song was sung by other
performers?

2. How come they used the first season's opening credits in the movie?

3. In the closing credits,how come they didn't mention Brian's name in the movie?

If you can answer these questions,I will be so glad.

First,the theme song is sung by the cast of the movie. Professional singers sung it in the series.

Second,the credits was used only in the first season to put it in the credits in the TV movie.

And third,Brian's name was referred as "Chris # 2" in the closing credits.

Don't forget to watch "Come On,Get Happy: The Partridge Family Story" Tuesday at 1 p.m. on VH1.