Ideally, it is best to do both projects at the same time, but you can do either project first if you cannot do them simultaneously. If you decide to replace your windows first, there are no issues. If you decide to replace your siding and trim first, you should not “wrap” your exterior window trim. Replacement windows are installed from the outside of the house, and if the existing wood windows have trim already covering them, they will need to be removed and reconstructed before they can be installed.