I sympathize with you about this since once upon a time I ruined 2/3 of an afghan with dye lot troubles!!! At the time I wasn't familiar with this method so all that knitting went to waste! Bummer!
BUT, this might help you, especially if your jumper is done in stockinette. As Crycket mentioned, alternating rows done with both yarns for a few inches will often hide the change. But what to do when you've gone way past where you joined the new lighter yarn?
Well, take a deep breath and get out those scissors!
OK, with a long needle (Circs work great!) smaller in dimension than the one you originally knit with, go back to a couple of rows before you changed yarns and pick up loops along that row. Then go a couple of rows above where you joined the new yarn and do the same thing with another needle. So you've got a space between the two needles where you added the new dye lot.
You're going to remove those rows. Cut across and unravel them. Working with the bottom needle start knitting alternate rows of the old and the new dye lots (with your original sized needle), which hopefully will camouflage the change. When you've replaced the rows up to the top needle, graft (Kitchener Stitch) the two rows together.
This might be worth a try. I'd recommend you check it out on a smaller swatch first, though.
Let me know how this works for you!
And happy knitting!