To be honest, I think it's one of those things that most fans would have been happy to accept as a "fact of life" - i.e. we all know Klingons look different in TOS because the budget was smaller, but in our minds we're happy to ignore that. Suspension of disbelief.
Unfortunately, the (great) DS9 epsiode "Trials and Tribbleations" ruined that - put Worf together in a room with TOS Klingons, and one of the characters is going to say *something*!!! So they had to come up with a canon explanation for it.
TBH, I've never watched Enterprise and never will, so I just assume they always had the bumpy foreheads. :P (and just put "Trials" out of my mind)....
No they didn't have to come up with a canon explanation they could have just left it as a mystery.
WORF: We do not discuss it with outsiders
Acknowledge it and move on.
Or, again, not acknowledge it at all so that there's no explanation needed. Don't even touch it or address it, treat the Klingons as if they look like present ones, or do as above and have Worf look like a TOS Klingon. But don't acknowledge that there's a difference because that opens the doors for questions! I think up until that point people had already pretty much chalked up the differences just being due to different production values and not indicative or any "real" difference. But DS9 came out and said that there WAS a difference which brings up questions.