Hi Gabriel,
This is a quote I copied and pasted from the FAQ section in Auctiva Help.
"If you have changed your eBay username, simply update that information in your Auctiva account by generating a new token.
"To generate a new token:
"Click on the My account tab
"Click the Ebay Settings link
"And click the "Get New Token" link"
According to this, you do not need to start over. I hope this helps.