ToUpperCases based on the original string

This works:

MapThread[If[EvenQ[#2], ToUpperCase[#1], #1] &, {Characters @ "05eccead24", 
          IntegerDigits @ FromDigits @ "1234567456"}] // StringJoin

(* "05eCcEaD24" *)

Edit: I first used StringReplacePart but that function is slow when used for many replacements, much as MapAt is. I have rewritten my code using a method from Map a function across a list conditionally and it is now orders of magnitude faster on long strings.

f[source_String, target_String] :=
  Module[{new = Characters @ target},
    (new[[#]] = ToUpperCase @ new[[#]]) & @
       StringPosition[source, Characters @ "02468"][[All, 1]];
    new <> ""
  ]

f["1234567456", "05eccead24"]

"05eCcEaD24"