Wow.. That was an interesting 5-page wild goose chase...
Let me guess, as soon as you switched to bloom, you started using bloom nutrients, then they started yellowing during stretch? THEY NEED PLENTY OF NITROGEN during the stretch phase. During this time, they are growing stem and leaf at a rapid rate, which depletes nitrogen reserves very quickly. Your Probloom (you talking about the Botanicare Pure blend?) doesn't have enough nitrogen to keep up with a rapidly growing/stretching plant, especially at 1/4 to 1/2 strength.
Your yellow, nitrogen leached leaves will not green up, but you can halt the spread by giving them some nitrogen.
The damage is done, but you can still pull through. Next time, taper off your veg nutrients slowly, even adding a little through week 3-4. This will keep your plant nice and green until near harvest (then hopefully it'll show some beautiful fall colors).
Good luck!