He just needs to download the license with the trial account, he doesn't actually have to download the whole game again.The download and expiration is tied to the account that downloaded it. When it expires you have to re-up it with that account.
IF you can get Okami on the new account, I'm guessing at minimum you will have to redownload it as the original download is tied to the current account. I could be wrong but I'd be surprised if you can re-up a game already on your hard drive under a new account.
Once you've activated the trial, initiate the Okami download, wait for the Activation screen to do it's thing, then cancel the download once it starts.