There's only one oriface tube but yes, the system must be completely evacuated before replacing it. The system should also be evacuated (vacuum applied) for at least 20 minutes using a vacuum pump before re-adding the freon to be sure that no moisture exists. It's a job for a shop with an AC machine if you want it done right otherwise you risk having moisture in the system cause problems as well.
I've never experienced or heard of a case where an oriface tube was partially clogged and causing your problem (besides reading it here recently) but I suppose it's not impossible either and might be worth trying. Just realize that it's a guess.