Combat logging most definitely did change between expansions. One of the most obvious example of this is how absorbs are calculated. WotLK does not log partial absorbs, and this was a very well known issue that was changed in Cataclysm, where partially expended shields would add to the shielder's healing amount. There's a reason why newer combat addons aren't ported to older expansions - it's not just the difference in API, but they outright don't have the data. But it doesn't work the other way around.
The thing is, from reading this thread I've seen no explanation as to why WoL actually handles WotLK logs incorrectly.
Consider the following:
We use a Cataclysm log in a WotLK-era parser: Clearly the WotLK parser will not support parsing partial absorbs and will give inaccurate numbers, assuming it doesn't crash in the first place.
We use a WotLK log in a Cataclysm-era parser: Partial absorbs do not get cataloged since they are not present, but we still manage to parse the entirety of the combat log. Which is what we wanted to do. The parse will be just as incorrect, but this is inherent to the log, not the parser.
There is no reason to believe that a newer parser would be inaccurate when using older logs. Any flaws it can have in parsing are simply inherent to the logs themselves, but they will not be misinterpreting the data given to them.
So far the only arguments I've read are "Different expansions, therefore it doesn't work." but I've seen no hard data to support this in the form of a log or a side-by-side comparison, instead these arguments lay entirely on statements like "it is known to be thus".
Can anyone give an example of a mechanic being incorrectly documented in a WoL report? Or ignored or misinterpreted?
Can anyone point to a line in a combat log and say "see this? WoL will interpret it as X when in reality it's Y"?