What are some shows that shortened their theme songs so that they don't credit their cast anymore during them, or shows that got rid of them completely? (BTW, the show had to have had a theme song that credited it's cast members before)

Got rid of:
Family Matters (Seasons 7-9)
Step By Step (Season 6; added it back in season 7)

Stopped crediting cast:
Reba (The first ten episodes of season two didn't credit the cast, but after that, they brought back the full theme song.)
Hope & Faith (Season 3)

Malcolm in the Middle - especially evident in S6. Half of those eps. that season just played the end of the show's theme song and only showed the cast' names within the episode.

Step By Step got rid of there Theme Song

The aforementioned STEP BY STEP is an obvious one.

WINGS had a theme song for its first, second, and half of third seasons. Starting with the episode "Marriage, Italian Style", the show got rid of the theme and started to do the credits over the show's opening scene. This is similar to what FRASIER did, but that practice was in place from the beginning with that show. One episode of FAMILY GUY didn't feature the opening theme song ("Blue Harvest"), and another didn't feature the usual theme but rather a parody of the "Naked Gun" movie's theme instead. MY NAME IS EARL doesn't do their long theme anymore (which wasn't that long to begin with--- about 30 seconds), but rather a short 3 second theme that simply states the name of the show (Jason Lee says "My Name Is Earl" over the created by credit). 8 SIMPLE RULES' original theme was about fifteen seconds, but after John Ritter died they got rid of that all together. NEWSRADIO, be it only for a few episodes, got rid of their theme in favor for a short introductory title sequence in which only the name of the show is introduced. The credits are featured over the opening scene. That's about all I can think of at the moment.

witch season did Wings stop plating there Theme Song

Boy meets world got rid of their them songs.

Malcolm in the Middle - especially evident in S6. Half of those eps. that season just played the end of the show's theme song and only showed the cast' names within the episode.
I think that only some Fox networks did that, because some episodes use the full opening in the later years, and cut out the cold open. And The first season cut the theme song for two episodes, and I know they used the full intro originally, seeing on my DVDs.

Step By Step got rid of there Theme Song
Yeah, I already said that! :wave:

There was always the more serious episodes of One Tree Hill previously that didn't use the theme song, but now it seems to be abandoned as of season 5.

"8 Simple Rules" ditched the opening credits as soon as John Ritter passed away, in the beginning of the second season.