Just looking for nice smooth shifting on the cassette. I don't know what that entails. Not interesting in cutting weight, just want smooth shifting. Will replacing the rear derailleur to an Ultregra be fine?

Current components are:

Rear Derailleur: Shimano 105
Crank: Tiagra triple
Brakes: 105
Chain: dont' know
Cassette: don't know

A well-tuned Sora setup will be smoother than badly adjusted Dura-Ace. I'd check with adjusting it just right before spending money on parts.

Park Tool's DIY pages for it:
https://www.parktool.com/repair/byregion.asp?catid=53
https://www.parktool.com/repair/readhowto.asp?id=64
https://www.parktool.com/repair/readhowto.asp?id=75

ANY components in their first 100 miles. After that, all bets off.

I'd suggest a tune up if you don't know what you're doing and don't want to mess with yourself.

If not, go with the park tools site that was linked earlier. Also, a tiagra setup shifts fine.

If you're shifting under heavy load, then that's avoidable and will cause many systems to stutter.

Here's where I'm coming from fellas. I remember riding my friend's road bike (used for racing) with really old Ultegra parts (possibly not even called that) and it shifted like butter. It's been 4 years and I've got $100 to burn or save.

This is the part where it's 80% the mechanic/20% components.


I personally would stay with the 105 RD... a new matching chain (Shimano Hypergride?), brand new cables/housings, then take yr friend to some fancy restaurant.

4 year old 105?
new cables/chain/cassette(if needed),maybe new front rings too. If you have a shop install the cables they should adjust the derailleurs for you too.