Well, for a start, I'm not at all sure that (totally non-electrical) issues relating to fire resistance (as legally regulated by Part B of the Building Regulations) are within the scope of an EICR at all. We are, after all, not talking about anything electrical at all, merely the possible consequences of the hole in a ceiling created to house a downlight. As I said before, if there were an electrical problem they would be able to specify what regulation in BS7671 ("The Wiring Regulations") they claimed was being violated, and it seems that they can't. Again, I don't think that non-compliances with non-electrical Building Regulations should be considered by an EICR (although, of course, one would hope that any such problems would be 'brought to the attention' of the property owner).
That aside ... I hope someone else will jump in to confirm (or otherwise), since it is possible that I may not be up-to-date. However, as I keep saying, I did not think that their is a requirement (under Part B of the Building Regs) for any ceiling in a single-occupancy 2-storey dwelling to be a 'fire barrier' (beyond the baseline requirements for any ceiling), regardless of what sort of room is above it - such requirements only existing if what is above the ceiling is a different dwelling (i.e. under different occupancy). Can someone confirm, or otherwise, whether this is still correct?
As above, I remain unconvinced that replacement of any of them is actually required.
Also, have you actually discussed this with the prospective buyers? If not, it could be that they don;t want downlights at all, or wouldn't want the ones you might have fitted. If the first thing they would do after moving in would be to have the downlights ripped out, it would clearly be a bit ridiculous to you even think of spending money now to have new ones installed!!
Kind
Regards, John