I've been given this question in one of my tutorials, 440V, 3 phase in wye connection with a reactance of 5 ohms per phase, Ra = 0, at some load the generator gives 8.2 kW at 0.8 PF lag.

It asks for the armature current, excitation voltage and power angle, all of which i've calculated through P = sqrt(3) Vl Il costheta, then used the equiv circuit to get the excitation voltage.. then the question asks if the magnitude of the excitation voltage stays the same, while load decreases by 20%, calculate the new power angle, armature current and new power factor; this is the part where i'm stuck, I have the excitation voltage and new power supplied, but i'm unsure of where to go from here? is there a proportional relationship somewhere i'm missing between the power and current or?

 

You would presumably assume the generator is still supplying an "infinite bus" voltage of 440V (line-to-line) with the reduced load connected to that same bus voltage.
Have you drawn the phasor diagram? I would recommend this. The visualization of what geometric changes are involved to achieve the new steady state conditions may help you in working out the new current, power factor & power angle.

You might also need to investigate circle diagrams in a generator connected to infinite bus.