Needing a certificate of occupancy (or CO for short) for commercial use isn't linked to whether you need to replat AFAIK. The point of the CO is to make sure that the facility buildout meets all the requirements for the proposed use - parking, fire suppression, ventilation, etc., etc. What a restaurant needs to have in place is different from what a boutique would need, which in turn would be different from office.
I'd get a design professional involved before dropping any serious bucks into anything beyond planning.