No reason to believe pearls changed, the brand's policy might have: after all, the seller doesn't promise that their merchandise will be worth more later or that they will maintain their price structure, or whatever.
Besides, there are so many more akoya pearls out there every day. No wonder even a top brand needs to acknowledge that their label in not irreplaceable after all.
The treatment of the day (overtone, say) comes and goes with fashion and customer surveys...
IMO.
However, the price you paid will stay on record, if you ever wanted to sell them (there are some papers with it on, no?). And potential buyers of your vintage Mikimoto pearls may wonder the same and find their own answer that may equate purchase price with value as often as not. Sure, signed estate jewelry comes with enough discount to make the point moot... mostly.
Other than that, they are worth what they are worth to you. Few jewelry is excepted from disappointing re-evaluation after purchase. Aside some rarefied items with very patient owners, everything is way more expensive to buy than to sell.
If memory serves...
PS:
It really is funny to see brands trashed and pumped on the Net. Is there a forum with Miki fans, Zeide? There seem to be enough out there, or mustn't' they speak with each other? 