There is a significant difference in table schemas in GP10 vs GP8. It is designed to integrate with Sharepoint and Office 2007, so is very denormalized.
When we upgraded from 9.0 to 10, it took 18 hours to convert the tables and do the upgrade. Our database was 32GB in size, so fair warning there. We were told it would take 7-8 hours by our service provider, then when I attended a GP "seminar" at Microsoft, found out that my experience was not uncommon at all and that some upgrades took even longer.