What is involved in swapping a 4L60E for a 4L80E in a GM truck?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

Transmission control

• The official HP Tuners walkthrough uses a 2000 Chevrolet Tahoe 5.3/4L60E converted to a 4L80E as its example application. (HP Tuners)
• For 1999-2007 Gen III truck/LS-style harnesses, the 4L60E-to-4L80E conversion guide moves the transmission-connector brown wire from cavity U to cavity S, and repurposes the removed white wire (cavity S) and tan/black wire (cavity T) for the 4L80E input-speed sensor. (Uploads)

Other wiring

• 4L60E/4L80E: Truck/SUV 4L60E and 4L80E transmissions have a factory reverse-light switch on the shift detent rod on the driver's side, and the same switch incorporates the park/neutral switch; swap harnesses wire their reverse-light wires to this switch. (Painless Performance Products (via Scribd mirror))

Confirm against the instructions for your exact parts before wiring. More: Transmission control in our tuning guide.