>However, they remain recessive for various reasons. This could include the recessive trait being advantageous in certain circumstances, but not others. Or that is only an advantage if other traits are also expressed at the same time

This makes no sense. "Recessive" described how an allele intersects with other alleles, and this is largely determined by the molecular mechanisms the protein produced by that allele is part of. A trait cannot change from recessive to dominant, no matter how advantageous or disadvantageous that would be. And while recessive traits are acted upon by evolution, that only happens in homozygous individuals that have two copies of that gene and thus show that trait. A newly-arising recessive mutation could spread through quite a few generations of heterozygous individuals, being passed on but not expressed, until two heterozygous individuals met and bred to make a homozygote. In the meantime, that allele could pick up new mutations that would change it's function without significant selection pressure.