The upgrade to Avante would just be a change in the license that you recieve upon activation, so you can simply reactivate the Laserfiche Server and you'll be on Avante. No other migration is technically necessary, although you'll also want to look into allocating your users into named user accounts. You can also check out this guide: https://support.laserfiche.com/resources/1550/upgrading-to-laserfiche-avante. It's a few years old but the information should all be the same. 

The Laserfiche Avante deployment guide (https://support.laserfiche.com/resources/3267/laserfiche-avante-10-deployment-guide) is also a good resource.