Vague Usage

For a new CS-Project the usage is not deifined finally. At present there is office and retail is planned. But, as the owner told me, there is also a possibility for a fitness center instead of the retail usage. We want to start the design review. I'm not sure whether it makes sense to wait until all usages have been chosen. What if we now assume a retail area and before submitting construction review a Fitness Center is fit out. The usage has influence for many prerequisites and credits (e.g. WEp1). Has anyone of you experiance with my problem?