Can I change any of the 5 finalists after the finalist phase has already began?

Absolutely. When you drag around the logos within the Ranked Logos box it will change who your top 5 designers are. By moving a designer up the ranks in the Ranked Logos box you can place them among the top 5 if you choose to.

For example, if you wanted to replace the designer slab04 with the designer FORZA, you could drag FORZA's logo in the place of slab04's logo. This is show below: