And really, they HAD to change the Klingons a little bit, and make them more honorable, because if they were all dishonorable, bloodthirsty betrayers like TOS Kor, wouldn't it violate the Federations' basic principles to ally themselves with the Klingons? I mean the very term Empire in Klingon Empire indicates that the Klingons take over world by force. So they had to tone down their villainy somehow, hence the code of honor concept.
See, the trouble for me really begins because it does lack consistency with the Klingons in TOS. Oh sure, the excuse could be made that the Klingons in "Errand of Mercy" had their own
interpretation of ethics and honor to which they were subscribers, or that as military
officers fighting against the Federation in a state of near war, they put aside those rules of conduct (or 'interpreted' them very loosely). But on the other hand, there's little indication that TOS Klingons care for honor at all.
I suppose (retconning like a man possessed) we might say that for the TOS Klingons, the scent of blood and the glory of battle temporarily supplanted their need to be honorable opponents?
Well first, Klingon honor is very relative to the klingon in question, and story being told. It seems very fickle, and what is and isn't honorable sometimes is very fluid and changing.
Also, are people in Western Civilization the same people they were 70 to 100 years ago? You can cite the UK and French Empires after WWII as examples, and even the US in its apparent downward spiral as a world leader. As a nation's world status changes, so does its attitudes and social mores. So why can't the Klingons have changed between TOS and TNG?
And actually, we did get some explanation for the Klingons changing (albeit somewhat a retcon) in TUC, when the moon Praxis blew up, that changed the entire Klingon economy. They no longer could afford their military budget, which means they shifted from an expanding empire to a world struggling to survive. And later on, the federation came to the Klingons' aid in both the battles of Khitomer and Narendra III. So, with the Federation helping the Klingons both economically and in battle, as well as opening up trade and cultural influence, perhaps the TNG era Klingons had some of the Federations' attitudes rub off on them, and they became more enlightened than the TOS era Klingons, at least relatively speaking?