Yeah, the carbonite freeze. It was shown in ESB as an ad hoc, unproven dangerous method of imprisonment. The bounty hunter that was introduced to this was Boba Fett. Now a new bounty hunter on the same planet with the same armor transports prisoners the same exact way. So...
...it's either a retcon or it's Boba Fett. Can be both. I can totally see Favreau hand waiving it "ah, they did it to Han, word spread, now they're doing it this way', which is fine, but why make a visually identical character do the same thing (other than nostalgia money ;)).
Except that it seems like a very logical explanation, even if he is a brand new character. Han was on display in Jabba's palace for months. Word definitely spread, and I wouldn't be surprised to learn that it's now a common technique used by Bounty Hunters to easily transport live bounties.
...which wouldn't be a retcon in the slightest. It's be a progression of the mythology.