Used this several times but never registered.. Glad to join the forum. First Question

I am updating some parts on my road bike -- want to swap a early 2000- 9 speed Campy Daytona Rear Derailleur with a 2005 - 10 speed Campy Veloce derailleur--will this work?. Everything else on my road bike is early 2000 Campy Veloce except the front derailleur which is also Campy Daytona.

I then want to use the 9 speed Campy Daytona derailleur for my commuter. My commuter is an old Concorde with a mix of old campy and miche. The only non-italian component is the rear derailleur which is a Shimano SIS (with bullseye sealed pulleys (not really sure what that means?). This is what i want to replace.

The front derailleur is old campy, downtube shifters are campy, Crankset - Miche 170 mm 42/52, Cassette - Miche 7 speed. Can I use the 9 Speed Campy Daytona with this mix?
Compatible?.

Is it easy to switch derailleurs?

Any sound advice appreciated. Thanks alot.