Cretak was also recast, as was Ziyal (twice).
Naomi, Tom Paris' Daddy and Molly O'Brien were all recast.
Actually, they used the same girl for Molly throughout. Maybe you meant Alexander. (I don't think Brian Bonsall was still acting when they recast for DS9)
I have no idea why they recast Admiral Paris. For Naomi, they probably wanted an older kid for a larger (possibly recurring) part.
With Cretak, the original actress was apparently unavailable. Supposedly, they wanted someone older for Ziyal, if she was to have a relationship with Garak.
With Neral, however, the character was so fundamentally different, I still don't know what was the point.