Can I replace the 2.9t engine with a 2.5t engine on the same car?

I strongly recommend staying with a direct fit part. If you don't, everything needs changed, engine, mounts, computer, wiring harness, vacuum harness. And there is a chance it won't bolt to the trans.

It is not possible. The T6 comes with a GM4T65EV all wheel drive with a chain drive unit making it too big for a T5 which comes with a AW55 and has an angle drive for the all wheel drive function. Also like Jacobandnickolas suggested that the harness will be different and the engine control module is set up for a 6 cylinder and not a 5 cylinder. The 6 cylinder comes with twine turbos and 3 oxygen sensors and different exhaust vs the 5 cylinder with one turbo and 2 oxygen sensors.