Yeah, that's not how it works.
I field more than 24 commanders at once all the time. I'm a bit crazy for it, actually.
The restriction that you can only recruit 1 commander/fortress/month is, indeed, a severe restriction - but converting them to non-commanders after they are recruited wouldn't help with that.
As niefelheim, you are very often in a situation similar to the one you describe. When that situation arrives, however, what most people do is simply stop using troops and field commanders exclusively. There are advantages and drawbacks to doing this, but if your commanders are flat-out *better* than your regular troops, sans equipment, this is what you'd tend to do.
Commander-ness is a feature of how recruitment is done, not of the unit # itself
.
(I'm drunk again, same schpeal)