I got a Cannondale CAAD 4 with campy old style 10 speed shifters, ultegra 9 speed cassette, hub, and derailleur. I took it to a shop to see if they could change the chain and cassette and they refused to touch it. They told me it would never ride correctly even after replacing the worn out parts. Even after I told them I knew the risks they still wouldn't do the work. Understandable - they probably don't want to deal with someone coming back for endless warranty work.

Obviously if I had known anything about bikes I wouldn't have bought it years ago, but there was a time when it was riding better and I want to keep riding it for a few more years.

The shop people told me I had two options:
1. Change shifter to Shimano, replace chain and cassette - about $500.
2. Change cassette, rear hub, chain, both derailleur, front gears to campy - about $500.

I don't want to spend that much. I was thinking about making this into a single speed bike anyway. Is this possible.

I thought there is a way to get campy shifters to work well with Shimano parts? A simple Google shows a lot of results so I'm not sure why the bike shop told me there isn't a way to get it to shift smoothly?

If it is shifting correctly now, and the only issues are chain and cassette wear, buy a shimano cassette lockring tool, chain whip, and chain tool and do it yourself. You can buy any Shimano or SRAM 9 speed cassette, and any 9 or 10 speed chain. I would do 9, as they are more durable.

If you have to do something with the rear derailleur, then buy a Sora or Tiagra 9 speed RD, and google hubbub derailleur installation.

Or, if you want to keep the current shifters, and get to 10 speed, buy 10 speed cassette and chain, the same tools, and a Shiftmate #3 from JTEK.

Take wheel off bike and bring to store and them them replace Cassette with a new one. But a new chain with a quick link while you're there. Done.

Like CNY said above... just route the rear der shifter cable like so. Your 10 speed Campy stuff should shift your 9 speed drivetrain like that. Unless the parts are actually worn out this is something you can do yourself with no tools. (Ok, a hex wrench, but you likely already have that.) I've got 9 speed Campy shifters running an 8 speed Shimano drivetrain with this routing and it is flawless!