That Highland Park 12 Year Old won't improve in the bottle once opened, though you may enjoy it more with each and every subsequent pour.
Like C57, I've not experienced any significant degeneration with opened bottles provided reasonable care is taken in storing them. But if you're planning on having any particular bottles around for years on end, it may be worthwhile to transfer the remaining contents to smaller bottles (and resealing those) once the level reaches 50% or lower.