WP
Even on the CRT I put out of pasture last year you could discern the difference between downscaled SD and HD. Try it yerself!
Personally, I ignore HD, I only care about SD. If you are stupid enough to shell out for a massive TV that you don't actually need (I'm happy with my 14 inch 4:3 CRT), then expect to see massive pixels or fuzzyness on old 4:3 TV programmes.
Even on the CRT I put out of pasture last year you could discern the difference between downscaled SD and HD. Try it yerself!