I think it's time to admit that after a month of trying to get the boyz to eat a healthier brand of food, that when it comes to cat food, like people food, healthy doesn't always equate to tasty.
For the past month or so I've been feeding the boyz a half and half mixture of their old food and the new food. Each of the three picks out the old food morsals and leaves most of the new food remaining in their dish. Thompson has noticeably lost weight and I suspect Diego-san has as well.
I figured after a while they'd give in to any hunger and start eating the new food. I even put out an extra dish to fill with the new food that is leftover. Theo nibbles away but the other two want no part of a new diet.
I do find it sort of amusing that after nibbling at their own portion, the boyz will switch dishes as if what I'm feeding the others is better than their own share. Thus Theo often ends up at Diego's dish, Thompson ends up at Theo's dish, and Diego ends up at Thompson's dish. It's the grass is always greener method of eating a meal.