Size change

If I had a voucher, would it be possible to change a halfling phenotype to a human one? Or vice versa, etc. Not a custom skin, per se, but a visual race change.

If possible, the change would be in appearance only, not mechanics.  Your halfling/fae would not be able to wield larger weapons, and changing to a halfling/fae appearance from human would not give the benefits of being smaller.  Plus, as anyone with a brownie appearance can attest, there's a very good chance anything designed for customization (quivers, aligned eye glow from spells, horns, horse mounting, head customization, etc.) would not work properly...many of them are designed around race/phenotype combinations, not appearance.

Best bet: If you want a character to look like a fae, make a fae character.  If you want to look human, make a human character.

VFX quivers work... The size/placement is not perfect, but if your base racial type is halfling or gnome, the offsets will be relatively insignificant.  Same with other torso aligned VFX's.... they'll be a big bigger, and the alignment will be slightly off, but nothing "broken looking" per se.

Head VFX's will not look right at all however.

Iouns will still look aright... you might notice some of the iouns dipping under ground on the lower side of their orbits, but you have to watch real carefully to see that happen.  Iouns are the only head-aligned VFX that will work for brownies simply because the VFX's do not really require close fits with the head model itself.