Yes, I suppose if we're going to get specific, calendar late October 2010 is when the entry-level AMD-branded 6000 series came out. The 6000-series still used the VLIW architecture of previous HD SKUs. The first major architectural change after the merger was RISC-based Graphics Core Next of the 7000-series introduced in late 2011/early 2012.
HD6000 was essentially the end of ATI's roadmap, HD7000 was the beginning of AMD's. That's how I interpret it anyway. ;-)
I'd say that going from VLIW 5 down to VLIW 4 in Cayman was also a pretty major change in architecture.