Ah, but there is. That was the initial feature of my editor (link in sig), way back
in 2009 (but use the editor instead, various small improvements have been made over the years). Open two BIQs (ignoring the warning that opening two BIQs isn't recommended), choose the section(s) you'd like to compare, make sure the "English" box is selected, and export it to a file. It'll show the differences between e.g. units with the same name, but also the new units such as the Grenadier. ("English" means where possible it will say things like "Upgrades to Knight" rather than "Upgrades to 42" if 42 is the index of the Knight)