The 80-year-old will reprise his role as the unscrupulous oil tycoon in TNT's forthcoming revamp of the classic CBS soap.
"He hasn't changed any," Hagman told Collider. "I'm very happy with that. They're writing in that direction, too."
The actor added that the new Dallas will focus extensively on JR's relationship with his estranged son John Ross (Josh Henderson).
"Well, we're both learning about each other," he explained. "This is the discovery period, right now. He's learning more than he wants to know about me.
"After all, his mother took him to Europe and educated him over there, and I hadn't seen him in 10 years, so we're both discovering each other. I know who I am, but he doesn't know who I am. There's that going on, too."
Hagman's co-star Patrick Duffy (Bobby Ewing) previously admitted that he is nervous about the new revamp.
"If we do it again, there's a chance of screwing it up," he said. "And if this new show fails, the whole world will hear about it."
Dallas will premiere tomorrow night (June 13) in the US and will air on Channel 5 in the UK.
> Dallas to debut, Falling Skies to return in June on TNT
Watch a trailer for the new Dallas below: