Dimension units not working?

In Main tab of Configuration > System I set my dimension symbols = in & weight symbols = lb but in the product page it still displays cm and kg.

When you save a product. It saves the dimensions and weight units along with the actual dimensions and weight in the product information.
Which means that you will have to save each product again with the new unit so that it's taken into account.

Ok, So first I need to delete my old product listings, then create new ones and the dimension and weight units will be fixed?

You don't need to delete anything. Just edit your product, set the new units if necessary and save.