I would say that Haig's style continued to evolve. He wasn't a pianist like, say, Barry Harris, whose style stayed rooted in bop. For example, Haig does an interesting version of Cedar Walton's "Holy Land" on Invitation. Given Haig's approach, I can see how he must have appreciated Walton's sense of craftsmanship and logical construction. Haig doesn't necessarily play funky like Cedar can, but Haig definitely was listening to & incorporating post-bop developments.
And I agree with you that Haig's way with ballads is very impressive. That didn't change.
I have several Haig LPs & CDs from 70's -- on Seabreeze, Spotlite, Choice, Progressive, and Interplay. All of them are well worth hearing, but Invitation and Ornithology (Progressive) are the two that I pull from the shelf most often.
EDIT:
Thinking about Haig prompted me to queue up this album:
Various Artists - I Remember Bebop (Columbia, 2 LPs)
This session is more bop-oriented than most of Haig's recordings from the time, since it's intended as a retrospective. (Haig plays four Dizzy Gillespie compositions here.)
From Henri Renaud's liner notes:
"And Dizzy himself confided to me that in Chicago in 1976, during the making of a TV broadcast about the history of Bebop, he was struck by the discovery that his former pianist played even better than in the heyday of Bebop when they both used to work together at Berg's in Hollywood!"