Hi, Antonio.
If u don't want to cancel an existing unit, try to modify, when u are opening an OOB, the CEH value. You will see u can add units without deleting the existing ones. If u want, u can also: save all, and when u reopen it, re-establish the same CEH (no modifications; the OOB will be the same you have saved).
For the OOB of La Corogne , first version, in the Peninsular Mod we are preparing, I've done so, and this has allowed me to build a OOB without Corps but only Divisions; modifying the initial CEH value I've added as many units as I wanted, and even of great quality like Guards, without the constription to create a new Corps or Division or to delete a unit.
Bye
Marco