ORIGINAL: stubblejumper
I never change any component without reducing the charge and working the load up again.Different cases have different volumes and a load that was developed in one batch of cases,may not even be safe when loaded in another batch of cases.
Stubblejumper is correct. Have you compared the capacities of both brands of cases? There will almost certainly be a difference between the two brands, and this could very easily account for the difference in accuracy.
Generally, Remington brass hasless capacity than Winchester,
but not always! So you should do a capacity comparison. Then redo your load workup with the Win brass.