Pete,
I've certainly done similar migrations and have just used the Export / Import facility in the ASM gui. If I remember correctly it will ask you to re-confirm couple of options applicable to each policy, Application Language for example. In v11 it has a nice Overview - Summary feature which highlighted things I needed to look at / amend in the new policy e.g. one feature that was configurable in v9.x but replaced by another one in v11. Can't recall if v10 has this feature, hopefully so. As you're migrating to the same ASM version then, you would hope, it would be a smooth import.
For good practice I'd make sure the Attack Sigs were at the same version on both appliances too.
Hope this helps,
N