My father used to work for the Gas Board when they put their livery on the vans they too removed all the badges, this enabled an neater job to be done (less masking out etc). They were not replaced to save time and cost as well as ruining the overall finnish. This will most likely be the same as for UPS.
Im not too sure i suspect that they are originally based on a certain vehicle(drivetrain ,chassis ,axles)then given bespoke bodies and cabs by a bodybuilder(or in house). the european versions look almost identical to the native american versions,and the cab area looks decidedly fibreglass based or G.R.P.have you also noticed that you never see ex ups vans?you can tell ex post office,BT,or other couriers vehicles,but you never see a re-liveried ups vehicle.