When a unit has a transom, the height is grayed out, because Softplan doesn't know which portion you want the height changed on. To change the individual units, you need to edit each of the portions of the combined unit in the Drawing Library ... Openings. Select the relevant opening, Click on the part that requires changing and press EDIT INDIVIDUAL COMPONENT ... you can then edit any of the parts of a combined unit independently.
You can place them all independently, but on the whole they function better as a combined units.
If they are units that you think you will use again, then place them independently, then EDIT > UNIT OPENINGS and give them a PRODUCT CODE and they will be added to your SYSTEM LIBRARY as combined units. Since this is the SYSTEM LIBRARY you may want to add a CUSTOM FOLDER (or FOLDERS) to keep them where you can find them again.
It may seem daunting at first editing within the drawing library, but once you get used to it, it's just as quick and easy as editing any other way, and much more powerful. It also gives you more control of grille placement/visibility.