I didn't really like the style of the TV series, 
except the cat girls, but I don't like the movie 
style any better- I think Hitomi looks like a fat 
Sailor Mercury, and Merle is godawful ugly.
Changes in character design are usually bad - 
such as the El Hazard and TM examples (My poor 
darling Ryoko in TMIL2 - she's gone anorexic!!!)
But they are very common - Devil Hunter Yohko had 
only 2 episodes with the same character designs 
out of 5 - I think it is partially because they 
came out years apart, and they try to modernize 
styles-
The only one I have truly loved a change in is 
Utena movie vs. TV- couldn't care much about TV, 
but I love the movie character designs. FY OAV 
are nice too. 
AIC seems to have gotten really bad- you could 
trade characters between El-Hazard AW, Tenchi in 
Tokyo, and BGC 2040 and they would seem to fit 
in- though for some reason I like some of the BGC 
2040s.  
Plus, I'm sure the designers probably quit, move 
on, get replaced, or get bored, and that probably 
contributes some.