If you already have a bottle dynamo, you know the advantages of always being ready for riding in low visibility conditions. You need to ask yourself why you're thinking about going to a hub dynamo?

If you're riding brevets or other long rides at night, the lower drag will probably be worth it. Go all in, with a Son hub for the lowest riding resistance. Prepare to spend big bucks. It'll be worth it.

For commuting, you need to ask yourself how long your commute is and whether it's worth the cost to change over. I'd guess you'd be looking at a Sanyo or Shimano hub, since drag isn't that important compared to 3-10X cost differential.

Either way, I'd suggest getting a new wheel. There's a good chance you'll need new spokes, since Murphy's Law won't let you find a dyno hub with the same ERD as your current wheel. Rim cost will be lost in parts and labor cost to swap the existing rim over to a new hub and spokes. After 20 years, there's a good chance your rims have been worn by braking and will need to be replaced.

You might just want to get a new light. The newer lights are SO much brighter than the older generations, it's amazing! And if most of your riding is in the day, your current wheel exhibits lower drag without the bottle dyno on than even the best SON hub.