No one knows for sure. If you change it to 18" you won't be the last person in line, since your order date also gives you a priority. If they continue to NOT make any 19" then you're better off switching. No one knows if they will start making 19" it's been only 18" for a very long time.
Ninja Edit: Just noticed you said May 5th order. Lol the 19" didn't even start so you're going to be waiting for a while since there's a bunch of people since March waiting for theirs.
Thanks! I just changed my order... from 19" to Aero... I wonder if i can buy these 19" wheels from Tesla afterwards