Did you export/import to migrate?
http://www.gotfusion.com/tutorials/tut.cfm?itemID=16
http://www.gotfusion.com/tutorials/tut.cfm?itemID=17
Any new version tries to be as compatible as possible with the older releases but in order to improve how the product functions there is always the possibility of issues arising on some site projects (depending on the site's complexity and other factors). Sometimes the issues are small, sometimes larger.
With small sites (under 75 pages) it is usually best to reconstruct the site again in the newer version and only start new site projects in the newer version.
It has always been this way with all new releases of the product. Sometimes you get lucky. Sometimes it requires a little more time and effort to move to a new version.