That answer was 99% correct... however, after an early decision rejection (or really, at any time during the cycle) you can:
1) Add the MD/PhD essays if you decide to apply to a program MD/PhD.
2) Add and send more letters of rec. You can send different letters to different schools.
3) Add another MCAT date/score.
You can't change your primary essay or MD/PhD essays once submitted. You can't change classes, your activities, or biographic info.