Replace.
substitute
One possible substitute for "their" is "his" or "her" depending on the gender of the person/people being referred to. Another substitute can be "its" when referring to a non-human object or entity.
surrogate or stand-in
duplicate, copy, model, substitute
Substitute.
Yes, the word 'substitute' is a noun, a singular, common, abstract noun; a word for someone or something that takes the place of another. The word 'substitute' is also a verb (substitute, substitutes, substituting, substituted). Examples:Noun: Salsa is a good substitute for tomatoes in a sandwich.Verb: Ms. Lincoln will substitute for Ms. Washington while she is on her honeymoon.
backup, temporary, another, alternative, substitute
Accomplice
The word "substitute" means to put in place of another, often referring to replacing someone or something with another option. The prefix "sub-" means under or as a replacement.
Substitute/Sub or Interchange
Yes, not only CAN it be, that is exactly what it is fore.
extradition