Yeah, not possible... but also wouldn't be necessarily be a downgrade in terms of cores.
6 full cores vs. 4 cores + 4 hyperthreaded, I'll take 6 cores.
I don't know how much more improved hyperthreading is in the newer cpu's but in the old p4 days, a P4 with hyperthreading vs a dual core P4, the dual core would of course win every day. Hyperthreading, depending on the application, was advertised to get you up to 30% improvement. Another core gets you 100%.
It is a downgrade in terms of everything else as the 7400 series is based on older architecture. No integrated memory controller, no quickpath, no sse4.2, etc.