In most cases celui can substitute qui and they can substitute quiconque.
Examples
Quiconque le sait, pourrait me le dire.
Whoever knows this, could tell me.
Celui qui le sait, pourrait me le dire.
The one who knows this, could tell me.
Howver, celui qui can substitute quiconque, only if they are the opening phrase of a relative clause. Without the definition by the relative clause celui qui can not be used for quiconque.
Examples
Indefinitive pronoun:
Quiconque peut apprendre à lire.
Everyone (whoever) can learn to read.
NOT: Celui qui peut apprendre à lire.
