I already have this situation and want to seal it up and put track lights. What do you think about using can light converter, and sealing up the old light can with spray foam and caulk?

It sounds like the can light converters depend on leaving the cans in for mounting support. The cans I've seen are often installed poorly with gaps around the insulation. It would be nice if you could check for that at the same time.

True. I was hoping to leave the old cans and fill them with insulation. I can't find any info or videos online that describe doing so.

I suspect the converters tie into the old can wiring therefore this has to remain accessible.