I am switch from bullhorn bars to riser bars. the length on my stem right now is 90mm, would you suggest getting a stem with a shorter length cause of bar change?
please let me know what you think.

I wouldn't, but it depends on what kind of position you want. Try them out first and see how they feel.

always cheaper to try before you buy! My guess is that they will work just fine.

okay! either way i have to get new stem because the riser bars i have right now wont fit the clamp size i have.
i am just trying to decide what length and angle i want.

If anything, I'd think you'd want a longer stem because the hand position on risers is less of a reach than on bullhorns.

Probably right around the same. If you think about it, your hands on the flats of the bullhorns are about where you'd want them on risers. Maybe a little longer but not much. You can get away with a stem with less rise because well, risers...

The thing about going to a longer or lower stem is it assumes that OP is trying to replicate the same position on risers as they have with bullhorns, which I wouldn't assume to be the case. It's just not really possible to say what would work best without more information.

I agree with [MENTION=25248]nightfly[/MENTION]'s suggestion. This is how I would set up my bike when going from bullhorns to risers.

90 is a nice mid size stem and should work out. You can always roll bars back or forward a bit to get it right. Same with the saddle