Let’s say a plane model file has been updated in UE/PE to add more detailing and this is apparently costing a demonstrable FPS loss.

If I keep the new .mdl file in the PLANE_Sqd and PLANE _QC1 folders but use the old .mdl file in the rest of the PLANE aircraft folders will it just be me with the new model and everyone else in the old?

Plane model changes (especially NPC plane models) were not so significant to have any impact on FPS (game performance). So, your changes are simply waste of time.

That was not the case when UE was brought into”service” and then using the older .mdl files especially for the Dr. 1 but okay thanks.
Fishing around at this point. All of a sudden (with the dozen variables in play) having FPS issues.