What is a mortgage account number?

I have a mortgage with the HALIFAX, I have a mortgage account number which I make payments against the loan to pay off said loan, all straight forward stuff.

The HALIFAX want to change the account number for my mortgage.

Questions -
Does the contract I have for the money I borrowed only apply to that number?

And if so, if they change the account number, does this modify the contract?

If they modify the contract without my consent, does this mean I don’t have to make payments to the new account?

Can I, in effect write to them and say, “oh I’m sorry, but I don’t have a loan with that account number!”......