If you're converting from R-12 to R-134a, you need to replace the expansion valve and the canister. You can leave the hoses and seals alone if you have no leaks. The old oil is left in, it will stay on the bottom, the new oil floats on top of it.
Also, don't get the cheapy kit that doesn't have it's own schrader valves. If the old valves are left in, the will leak sooner or later, and cannot be replaced. The fittings that you put on have a clear threadlocker on them (which can be removed with lacquer thinner) and cannot be removed without destroying the threads.
So, to convert you need to:
1. evacuate the system
2. replace the expansion valve and canister
3. repair any leaks
4. remove schrader valves, install new fittings
5. vacuum system check for leaks
6. Charge with R-134a. You will need to look up to see how much freon & oil to put in. Less R-134a is used than R-12, so this is critical. Buy your freon from a parts store, they can tell you how much is needed. Also, your pressure readings will be different.
