Interesting...though I'd want to qualify that maybe higher up. I'm concerned that salesperson didn't know what they were talking about.
I know the battery terminals for a 2nd motor are on all M3 batteries so you'd think it would be fairly easy to do unless it's just that Tesla won't allow it given they have control over the parts supply and modifying the software.
I thought I was able to edit my design yesterday and that included selecting AWD and Performance in which case you'd still be going off the same order/down payment just that it's date would change if you selected something different.
I've also heard you can "probably" talk to sales and if you wanted an S or X, (new or used) they would apply your M3 deposit towards it though again, I wouldn't take the word of sales for gospel on that either.
I'm kind of looking forward to RWD in the winter with R2's