Splitting a Customer account into many smaller accounts

SOLVED

I have inherited a set of accounts where we have major builders as customers.

The accounts are a mess. Each site receives their own invoices (but all through the one customer account). And each payment received is from the site, and again all the payments go on the one account.

I need to split the customer account up into individual sites so they are manageable. How would I go about this process?

They go back years, so not sure how I would even start.