There are two questions you need to get answered (and I don't have definitive answers)...

1. In the case of changes in FAA interpretations of the regs, is it possible to get "grandfathered" under the old interpretation?

I think the answer has to be yes for people who took their checkride prior to 2010 because otherwise the FAA would have to retroactively revoke their certificates.

2. Would said grandfathering apply to sim time prior to 2010 but a checkride after 2010? I doubt it. My guess would be that grandfathering would only apply for checkrides which had already occurred prior to the interpretation.

Also...this is not really an arbitrary change in interpretations. The old regs (long time ago) actually seemed to allowed GI's to give sim instruction. These regs changed years ago but some folks still kept doing things the old way. Eventually someone (Gatlin) got around to asking for clarification.

So I would consider your argument pretty weak, and probably not worth fighting with an examiner on checkride day...he'll probably just decline to give the checkride without clarification from the FAA. Save yourself the hassle and go get some flight time.