It's my opinion that when TMP was made, the fact that Klingons had ridges was just a retcon -- i.e., the intention was that Klingons ALWAYS had ridges, and the Klingons in TOS were smooth-headed simply because they didn't think of giving them ridged foreheads, or didn't want to go through the hassle and expense.
No matter what Klingons looked like in TOS, TMP told us otherwise -- that Klingons were always SUPPOSED to look like they did in TMP.
I'm old enough to have seen TMP in the theaters in 1979. I remember noticing the difference in the Klingons, but I also remember not really being concerned about it -- because it was just a movie, and TOS was just a TV show.
DS9 knew this in Trials and Tribblations
, and they were smart enough to intentionally stay away from the issue by addressing it in a very tongue-in-cheek manner (in a very tongue-in-cheek episode that was more a valentine to the fans).