Since there is currently no easy way to migrate over from redis to mongo... running the install on an existing installation is necessary to populate mongo (even just the administrative user)... So running setup is the only option to switch over at this point. Hence the problem and the reason I posted.
Alright, but the fact remains that before running ./nodebb setup, npm install has to be run first. Because npm install makes sure all of the requirements are met, there is no reason to perform those same checks again in ./nodebb setup.
Considering there is currently no "proper" way to switch between databases, you are right in saying that ./nodebb setup needs to be run to populate the database. However, it will not move over the already existing data, hence it would be easier to just install another copy of NodeBB with your new database settings in another folder and copy over the old node_modules folder from the previous installation afterwards. This should also solve the error you are having.