Do I need to change anything in my project when compiling it for the first time in VFP 8?Not much.
The main changes have been in the SQL sublanguage, to make it more ANSI 92 compatible. As Craig said, I would advise to change your queries instead of reverting with SET ENGINEBEHAVIOR which only masks a problem. In general the changes are not that difficult.
You will find that the compiler is tighter and does not allow a few things that it previously overlooked (like missing comma separators in LOCAL declarations) for example.
The menu structure has changed somewhat. It now allows for placing images in menus. If you use this capability, your menu files will not be backward compatible (i.e. can;t be opened back in VFP6). There is a way to restore them the way they were if you are really pressed. I would instead just test on a backup of your whole project.
Table validations have also been tightened. See TABLEVALIDATE.
As you see very few "issues" and all for the better. Go for it. (You should go for VFP 9.0, why jump to VFP8?)