Ventilation system - what to do if the room is planned to be repurposed?

Thank you for your help! That definitely helps us already! The appointment is also with the planner, I had expressed that incorrectly. Let's see what he says about it. But it is already reassuring that the pipes can be "simply" reconnected.

The question remains whether that is really absolutely necessary. Our office doesn't even have a ventilation system :) Bathrooms/wc and kitchen obviously have to ventilate, but something like that is planned from the start anyway

We have been thinking about whether or not to install a ventilation system for a very long time anyway. Now we are relatively sure that we will install one (among other things, to prevent mold caused by improper ventilation). We had also initially considered getting one only for the residential unit, and not for the office part. But that doesn't really make sense either. It is still unclear whether the office part might be rented out permanently. And whether the tenants will always ventilate properly...??

So you can simply switch the supply air and exhaust air on the device. And you can continue to use the same duct as before? That really wouldn't be a problem then.

Normally, ventilation systems are planned as follows: - Determining the air exchange rate, from which the required volume flow is derived with the room volume - Defining supply air areas and exhaust air areas (the overflow areas remain) - Distribution of fresh air to the supply air areas. Then follows -- I believe/hope -- some hocus pocus regarding pressure losses, flow velocities, [Vermeiden von] short-circuits etc. And the planning is done (usually, it will be found that the sum of all supply air flows is exactly equal to the sum of all exhaust air flows)! If you now connect a supply air valve at the central unit to exhaust air, the whole concept gets out of balance (Sebastian79 basically wrote that in #2). Therefore, I would write down the requirements and take them to a (competent) planner.

It is relatively easy to change the volume flows. Reducers are used, which reduce the flow until everything comes out as planned everywhere. Nothing else is done during normal commissioning. The shorter the hose to the outlet, the more would normally come out there. Nevertheless, you are of course right that you cannot just plug it differently. As I already said, it is probably easier to just leave it as it is.