If I were to plan out a progressive upgrade of drivetrain due to budget constraints, would that be even possible? Specifically, going from Sora to 105. (9 speed to 11 speed)

Are there any parts that could be upgraded to 105 first, or does it need to be done all at once?

I'm thinking brakes would be an obvious thing that could wait, but anything else?

repair it stay with 9 speeds .. unless the gear ratio you need a lot is not in any 9 speed cassette made .

its not free lunch . Speed, Velocity, ie the work you put out , not the parts pick.

and those 11 speed brifters, at retail, are expensive , the cassettes and chains cost more..

what do you expect to gain for all this expense?

maybe you will impress your friends with just a 105 9 speed RD upgrade? its the most visible..

Upgrade only those parts that are definitely broken, and stay with 9-speed. Upgrading one part at a time is far expensive than buying a new bike. 9-speed is more durable than 10 or 11-speed. Wait until you have plenty of money for buying a new bike of your choice.

edit: fietsbob types faster than me.

It might make sense to do this if the larger objective were to one day transfer all the 11-speed components to a new frame... especially if the 9-speed components are starting to wear out anyway. 105 brifters, rear derailleur and cassette would all have to go on at the same time, yeah that's a big price tag.

Didn't they even change the brake cable pull ratio for the 11-speed group?

Ordinarily, my thinking would be to try replacing the rear wheel, cassette, chain, and rear brifter, and see how everything plays together, but I'm not even sure that would do it.

I think brake cable pull ratio is still pretty much the same.

As far as shift though, definitely different. And I think even if he wanted to change the rear the 11, and leave the front as is, it wouldn't work out so well because of the narrower 11s chain and the wider 9s FD cage. You could probably get it to shift, but it would likely be at the extremes of what the FD could do. Not to mention 11s FD's have that stupid plastic bumper on them now too (thanks a lot Shimano).

Brake cable pull for all Shimano brifters using shift cables routed under the bar tape is different than for brifters having exposed shift cables. Shimano also changed cable pull ratio for 11 speed shifters and derailleurs so that the newer shifters will not work with either the front or the rear derailleurs from the older style groups. Added to that, the 11 speed cassettes require a cassette body that is almost 2 mm longer than the cassette bodies used for 8,9, or 10 speed cassettes. Upgrading from 8 or 9 speed to 11 speed is pretty much an all or nothing proposition
Dura Ace 7900, Ultegra 6700, and 105 5700 brake calipers are compatible with 11 speed brake levers but using the older Dara Ace 6800, Ultegra 6600 or 105 5600 brake callipers from previous generation 10 speed groups will result in a noticeable increase in the effort required to stop your bike, so upgrading from 10 to 11 speed could also be and all or nothing proposition