Some of it is from Toei sites. For example,
this article on the making of Agito, which explains the differences between how Kuuga and Agiro were shot and edited, and also says that shooting Kuuga in HD will allow them to make true HD masters in the future. Similar articles have mentioned the switch to HD for Boukenger, using RED cameras and recording on-set audio for Shinkenger, Wizard's camera upgrade, and so on.
Other info is from looking at the DVDs and BDs. On Carranger's DVDs, the first five episodes are digital masters of the original film, but the rest are from old tape masters. The episodes from tape masters still have film glue on the first and last frames of almost every shot, which means most editing was done on film. But there are places where the video does things that can only happen if there was an edit at the tape stage. Gaoranger-Magi are full of 30fps effects and digital composites, but there's still film grain on most stuff. Bouken-Go-On still have film grain as well.
Oh, and as for new extras we know of:
BLACK, like Faiz, has a new roundtable split into three parts, with one on each box.
Kabuto's BD box listings don't show anything new.
Blade also has a new roundtable, but it's split into two parts included in boxes 2 and 3, according to current listings.
BLACK RX's listings say new interviews are planned.
W, Shinkenger, and Ryuki didn't have anything new with their BDs.
While these aren't new, the Hyper Battle Videos for Ryuki and Faiz were previously only available on VHS. (Agito's HBV is the last one stuck on VHS as of now, since Kuuga's is in a DVD volume)
BLACK box three has Toei TV Hero Encyclopedia 1, which was released on VHS and Laserdisc in '93, but never had a DVD release. (You may be familiar with Toei TV Hero Encyclopedia vol 2, which was included in the last Jetman DVD volume)