lingerbw...
Think about getting it evacuated twice. First time like you say is to get rid of the R-12. Then you'll have to open the system to do it right. You want to drain out as much of the compressor oil as you can. You probably should replace the receiver/dryer. Maybe all new seals, because the type of rubber O-rings might be different for R-134a.

Then after you're done with all that, have it pumped down again. Actually it's best to hold it at a hard vacuum for a couple hours, because it takes forever for moisture to come out of the system. Moisture that's still in there will kill the performance. Then add your R134a & retrofit-oil.

If you don't do all of that, it might just work OK. Depends on the humidity in the air when you open it. Depends on how long it stays open. Depends on how much time passes between opening the seal on the new receiver and evacuating. All that stuff you can't control, that's why some people will say they did it and it worked OK. Other people will tell you that it won't work because it didn't work for them.