I think the card validity should define the airport services as at least in case you drop out of a "perk level", lounge access etc. will still be valid until card expires. BA had one month grace period and all airport services were valid during that time. In a sense, it is a same situation as crediting the points to another FFP but using the card to gain benefits from another.
Tier status on BP is not the only defining factor. I guess the main issue is for e.g. OWE->OWS transition that how long one could access FC check-in and lounges. Actually, equally good question would be can one access PW during the grace period? AY could be more strict on the matter if they want as from AY perspective customer is not a platinum member anymore.