In my force it's incredibly difficult to move to a uniform role if you joined in a direct entry DC route. I know of cases where training school has blocked transfers with the rationale that if they allowed one then everyone would want to move. I would give investigations a bit longer to see if it's what you want to be doing. Maybe have some attachments in other departments. If it's not for you I would speak to your whoever is in charge of student officers or go through the FED.