Hi
"There is a realisation that changes must be made."
Sorry, in my previous post I took liberty with your original sentence. I was not attentive enough.
"Is that changes must be made a noun clause and a complement of "realisation" or a noun clause and an apposition?"
Apposition and relative clause, but not a complement.
that changes must be made is in apposition with realisation
"What type of clause is that changes must be made and what does it modify?"
a noun clause, and it modifies realisation