Good question !
Some scholar should take it upon himself to discuss it further. Unfortunately I am not a scholar. It just crossed my mind while reading the post of Abu Muhammad....
I had always had my own secret confusion about one other aspect of it. It is in ahadeeth that some awlia will find Syedena Mahdi in Madina Munawwara and ask him about him being Mahdi. Syedena Mahdi will refuse and go to Makkah; they will follow him to Makkah and approach him and he will escape from them back to Madina. That will happen many times...
It used to puzzle me as to why would Syedena Mahdi refuse to acknowledge himself being Mahdi ?
This Hadeeth gave me at least one possible explanation. May be before that ‘one particular Night’ Syedena Mahdi himself is not aware of him being the Mahdi ???
Eslah here, then, would probably mean that he would be granted those ‘special abilities and qualities’ befitting Mahdi, although he would be a ‘saleh’ Momin even before that night, he would be Muttaqi even before that night, but there is always room to be more Muttaqi.....
Well, that is just a layman explanation, as I said I am not a scholar. My word here should not be taken seriously !