Let me start out by saying I don't recommend doing this. Systems that were engineered for R12 traditionally do not perform well with R134. In some cases, it works but most of the time the condenser is too small, the fan is insufficient and an already tired compressor usually quits under the extra load. Some of the other problems is that when R134 is introduced to mineral oil (R12 oil) it turns to jello. This can happen in a place where not much is effected or it can cause a real problem. R134 is also made of smaller particles and can find a smaller leak.
Having said all that, here's your concerns and procedures. First you have to remove as much of the old oils as possible. That usually means removing compressors and flushing condensers and other components. An additional condenser fan will improve things. You need to charge the system with preferably "ester" oil which is compatable with both R12 and R134. Charge the system with about 75% of the original capacity of refrigerant and hope for the best. In other words if you had 3.5 pounds of R12 you would use 2.6 pounds of R134a. If you use 3.5 pounds you will be say overfilled

The R134a will give you about 10% less cooling than the R12.

I really suggest that you dump the R 134a and fill with three 6 oz. cans of Duracool (HC 12a). You'll have about 20% better cooling than the R134a and it will be compatible with whatever oil is in the system.

Your compressor high side pressure will be about 100psi or more lower. This will give less load on the engine and less wear on the system. It also will be less prone to leaking past the clutch seal as the molecule size is much larger than R12 or R134a.

Emery Stira
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...You cannot add ester oil unless you complete drain and flush all mineral oil. This would also require changing the receiver/drier.
Emery Stora