Hi Virginia: A suggestion to your first question about changing the invoice or balance -- if you entered the invoice as a 'historical invoice' could you not just enter another one and make it say whatever you need it to say to correct the first entry.
As for the second question, I'm not sure how historical data works when entering in the A/P module or A/R module. Would you have to actually enter the historical info through the A/P module in order for it to be included in Accounts Payable. Can't help any further on this question as I have never started up a set of books by using the historical.
By the way, great answer to the question from the fellow about buying customers. That's what I would have said too. I was thinking about the situation he had and didn't that kind of purchase of an "intangible" get set up in the asset section as "Goodwill"? Cheers, Rita