I have a guy telling me that the Accord chassis has been almost the exact same since 1990, just had tweaks and minor modifications on the same chassis over the years to make it more rigid and such.

Is that true? If not, can you please get me some info to show him to prove otherwise?

He's a mechanic and claims that he's seen plenty of Accords from underneath and they all look essentially identical in terms of the chassis from 1990 onward.

I really really doubt it.

He also says the Camry has been unchanged since 1992, though I'm pretty positive that it was redesigned from the ground-up (including the chassis) in 2002, at least.