Most ECU's handle altitude changes really well. It's a fairly simple mathematical calculation based on the air pressure reading from the sensor. A good ECU will have a dual sensor, so it measures manifold pressure and ambient air pressure as you drive.
Cheaper set ups will have a single sensor. It measures ambient air pressure at start up, and sets the fuel map based on that. If you're climbing Loveland pass, it will run great when you start the engine at the bottom, but will gradually get richer as you ascend.
There's always an Old Guy who will insist that a carb is better than EFI.
