The road bike I just bought has the Shimano Tiagra 9-speed STI shifting. The shifters are first generation Tiagra, they have the index finger operated paddles for upshifting and downshifting (no thumb triggers) and the shifters have the bolt that holds the shifting assembly to the clamp exposed as opposed to the newer Tiagra 9-speed which has a silver plastic cover over the assembly bolt.

I have noticed that every once in a while the right shifter which operates the rear derailleur will skip (as in not engage) when I move the larger lever to send the rear derailleur to a larger cog. I have a feeling due to the age of the shifters (the bike was new in 2002) the shifter is failing and I will be replacing it sooner or later. My question is are the Tiagra 9-speed systems compatible with the Shimano 105 or Ultegra systems as in if I buy a 9-speed set of 105 or Ultegra srifters will they integrate properly with the Tiagra 9-speed rear derailleur or will I have to replace the rear derailleur as well?

Thanks