@Vegard:
I'm pretty sure the series switched to HD at the same time as the widescreen-episodes started, only that nobody actually recorded them like that before episode 250-something. Looking at the fifth opening, it was clearly animated in HD, and I doubt they'd do that if the episodes themselves weren't also like that.
I don't think it was broadcast in HD at that point, but it was most certainly produced in HD at that point with a digital master produced. Japan made a more gradual progression into HD, they started by moving the standard to Widescreen (whether it was actually broadcast in HD or not) and you could actually get an "EDTV" which wasn't "High-Definition" it was "Enhanced Definition". Basically, it was the same definition as an SDTV, just with a Progressive Scan picture instead of an Interlaced Picture, and it was Widescreen.
SDTV (Standard-Def) - 480i (i = Interlaced)
EDTV (Enhanced-Def) - 480p (p = Progressive Scan)
HDTV (High-Def) 720p-1080p
One Piece was most likely Broadcast in ED from eps 207-260 and then it began airing in HD. But considering they moved entirely to Digital at that point, they were probably producing the show in HD and down-sampling it for broadcast (as they are now doing if you watch Fuji TV on a Non-HD TV)
So putting that on Blu-ray would be easy.
Now, for eps 1-206, that's gonna be a bit of a hard one. It'll most likely need to be upscaled. Wont be hard though, if Yu Yu Hakusho can get a Blu-Ray release, One Piece can no problem.
Now, anyone who thinks a Blu Release of One Piece 1-205 should be stretched or cropped to widescreen… needs to be punched... The show was produced in 4:3 up until that point, so it should remain. That is why Pierrot is leaving Yu Yu Hakusho in 4:3 "Pillar-boxed" on their Blu-rays, and that is why Dragon Ball Kai is released in 4:3 on the Blu-ray disc release. If you want the show cropped or stretched, you just hit the stretch or Zoom button on your remote, and it does it for you, that way everyone is happy. If Toei stretches it or god forbid CROPS it, they'd piss off... a LOT of people...
So, when OP eventually gets a Blu-ray release (which SHOULD Happen... if YYH gets one, One Piece damn well should...) it should look great in HD, much better than DVD, but not truly glorious until it hits Ep 207, and then it'll probably look LEAGUES better than the "HD-Rips" we have now, which are probably victims of ripping and encoding issues (I remember Elric complaining a lot about the colors and encodes of some HD-Rips)