Doing any kind of conversation with the new converter turns all existing materials (from previous corona versions) now named corona legacy materials to the new physical material. The only way to avoid this is by doing the "convert selected". I would consider this being a bug.
I wouldn't consider it a bug if there was an option where you could select "don't convert legacy materials to physical" though. Why didn't we get the convert to legacy material feature as well?
Maybe it is me but some of us find it hard to do the switch from legacy material to physical material overnight. Besides the fact that all of our commercial projects from before were based and made with the newly called legacy material. I personally get a lot of requests from clients for updates on older projects. We are opening older files done with corona 5 or 6 and we have to be super careful with using the converter in those files since it might turn all of the already made materials to physical which then results in a completely different image... And I kind of find this frustrating to be honest.
I think that if you plan to make such a radical leap, and I do find it radical for the above-mentioned reasons, you should at least educate your users and provide video tutorials on the official corona youtube channel where you break down the differences between the legacy material and the physical material. Where you actually re-create the same material in both legacy and physical format. This way the user will find it easier to make the transition and actually not run away from using the new features.
My 2 cents...