If the system is still working well, you get the "plan ahead" award for this one. Your system could go years and years before needing attention.
When the time comes, you will be much better off to repair whatever leak allowed it to get out, evacuate and stay with R12. Even at $29 per can it is still the least expensive major component in the a/c system.
There are two very valid reasons that you MUST evacuate an empty system before charging:
First, if the system has air in it, then there is not enough room to get an adequate amount of refrigerant in the system with the air. Remember air is basically a gas, without evacuating that gas, you won't have enough room to get in enough of the gas that you really want in there (refrigerant.)
Second, this is most important, air has moisture in it. Moisture combines with the refrigerant, any refrigerant, to make an acid. This acid will eat a hole in the evaporator, condensor or anything else it can find.
There are many other elements of a 134 conversion that you may not have considered. When changing, you have to change the oil to a 134 compatible oil, you have to put an XH7 or XH9 dessicated filter/drier in place. To change oil, it is best to tear the entire system apart and flush thoroughly, then put 134 compatible o-rings in place.
When you are doing this you are most certainly seeing to some other problem such as a leak or a bad compressor. It will cost the same to fix that problem whether you are staying with R12 or not. The trend is seeing R12 go DOWN in price. There are fewer cars on the road every day that require R12, decreasing demand, thus decreasing its value. By the time you have to pay some attention to your system R12 may very well cost less than it does now.
You must look at the big picture. Many people can only see the cost of the refrigerant alone and end up spending more time and money changing a system only to find themselves disappointed.
When the time comes I can promise you that you will be happier if you stay with R12. Most MB air conditioners just don't have the reserve capacity to tolerant the efficiency loss. R134 will only have 80% to 85% of the cooling capacity of the R12 system. Your system just was not designed with this much design margin.