Depending on the style of your chicken tractor, you can probably just move the heat lamp outside when the chicks go outside. You might want a tarp to cover part of the chicken tractor for a few weeks, too. (So about half of it is free of wind.)
The chicks will need one warm spot in the tractor, just like in their current brooder--the rest can be as warm or cold as nature makes it.
As they grow more feathers, they will spend less time in the warm spot, and more time in the other spots. When you check them at night in the tractor and find them all sleeping away from the heat, then you'll know it's time to remove the heat lamp. That will probably somewhere between 3 weeks and 8 weeks, depending on the weather and the particular chicks.