Anyone know if this can be applied to existing ad codes via editing in the control panels?
I don't think so. When you switch from one to the other, the code in the box changes completely. By coincidence, because I was redoing a couple of ads' sizes (due to the change that they will now backfill larger ad spaces with smaller ads) I saw this and updated all the ads on my site with asynchronous.
While I have not had any problems in the past 5 days, I have also not seen any appreciable improvement in loading speed or bounce rate.
I'd think if you use the same channels, you're probably ok
It has been my experience as well that this is true. I never have any problem changing ad sizes or codes as long as I use the same channels.