I think the differences in the art between the early TV
episodes and the OVAs are simply due to Studio Pierrot's
style evolving (or devolving, depending on your opinion).
If you look at Ayashi no Ceres (a new series from Pierrot,
also based on a Watase Yuu manga), the art style is just
about identical to the FY OVAs.  

One thing I did like about the OVAs is that the art
was at least more consistent.  In the TV series, the style
would often change from episode to episode.  In general,
though, the art in the TV series is rounder, and the art
in the OVAs (and AnC) is more jaunty and angular.