First I'll just say that I get why they maybe wouldn't want to do this in any extensive manner as people would absolutely abuse it by buying this for the trial, "returning" it and then just do it again. That said I think it would be a great feature and it seems plausible that a short trial period of say 12-24 hours in real time could be implemented per account with a long cool down. So lets say once a month you would be able to do this. Making a race change can be a big deal, it's not exactly cheap, and PTS just doesn't have the player environment to really test out how a new race will perform. Stats dont always equate to performance.
Just an idea that I for one would really enjoy and Im sure many others would as well.