If you might want to use the edits in future, filming and editing in HD makes good sense with downsampling to Flash etc after edit.
But if the edits are never going to be used again, then downsampling before edit, maybe to DV, might ease workflow with current edit software.
A free utility like Virtual Dub may be able to do the conversion. My HD work so far has all been with M2T files off tape. I haven't worked with AVCHD.