I've never used QuickBooks, but for some reason, I've always assumed it was almost directly comparable to Peachtree. Anyhow, Peachtree upgrades have been completely straightforward.
For what it's worth, I would do all accounting program upgrades from disk, even if I had to pay a premium. Perhaps just a paranoid personality at work, or maybe slow internet connection.
If you haven't bought the upgrade yet, you might want to check with Staples office supply. On Peachtree Complete upgrades, you can save $200 or more over the cost of ordering from the software house. I'm not sure you would go this route if you have a networked version, but for single user, it makes really good sense.