First sorry about my poor english. I'm not an expert, but some tech papers I have about these comps sugests retrofit using from R22 to R407c with following precautions:
lubrificant for R22 is mineral origin and r407c is sintetic (poliol ester or POE). You must be sure of a good refrigerant cicle cleaning before charging the ref and the new lubf. Attencion: lubf contamination causes compressor bearings failure. I really don't know anything about seal. Where are you from? Does your quest was send to Hitachi Japan or US?
It was my first time on site, and I thought that most of forum pals were from US. I'm sorry. I know there is a HITACHI plant at Barcelona.
About my tech papers I think it will not usefull for you. They were written in Portuguese (I'm from Brasil).
Compressor Series 2 in Europe always uses sintetic poliol ester oil (SR30). Recently, in Series 5, they changed for Castrol Icematic SW220HT (desevolved for HITACHI only).
There's no graeter contamination risks between them. A good vacuun it's enought. Consider possibility make your gas retrofit by the overhaul stop.