To go to a lower rank is easy, but to go upwards then you need to achieve this (for example a Sqn Ldr looking to become a Wg Cdr):
Must either have been, or currently be, a Wing Commander or a Squadron Leader eligible for promotion in accordance with AP 3393 Vol 1 and AP 7000
So the criteria is the same, but the promotion board competition will likely be far fewer - most FTRS posts have fewer than half a dozen at interview.
LJ