Say I have circuit board -- e.g. audio preamp board -- that mostly uses 1/4w carbon film R's. All else equal (e.g, same resistor manuf, and similar manuf dates), will substituting 1/2w R's increase, decrease or make little difference in Johnson noise?

What other beneficial or deleterious effects might one encounter by change of resistor wattage?

 

The increase is wattage rating is likely to have little effect on the Johnson noise unless the resistor is dissipating enough power to raise its temperature significantly.
Otherwise it should have little effect on circuit operation.

Changing to metal film resistors could reduce the noise some, as carbon film can generate noise higher than the theoretical Johnson value.