Replace $n$-th occurrence with number $n$

How can I replace $n$-th occurrence with number n for string/for list?

For string :

StringReplace["amkwajefiwaeah", "a" -> "A"]"AmkwAjefiwAeAh"

But how can I get

"1mkw2jefiw3e4h" or
"A1mkwA2jefiwA3eA4h" ?

For list :

ReplaceAll[{a,m,k,w,a,j,e,f,i,w,a,e,a,h},a->A]{A,m,k,w,A,j,e,f,i,w,A,e,A,h}

But how can I get

{1,m,k,w,2,j,e,f,i,w,3,e,4,h} or
{{A,1},m,k,w,{A,2},j,e,f,i,w,{A,3},e,{A,4},h} ?