Do you see the three flat blade setscrews under the center column base? If so, back them out before trying to remove the head. This is nice feature on Bogen pods that keeps a head from loosening up as you use it. The bottom side of the head base has facets that the setscrews engage.
It is quite possible the legs angle makes it impossible to set the bottom screws. Close it all folded, tighten the screws, then do whatever you want with the legs, afterwards. Or are you asking how to set the center column horizontally on the Pro's top base?
What version 3021 do you have? The recent N model has an adapter that you can unscrew from the bottom of the faceted center column which you mount in the jaws of the center column clamp. It then serves as a platform to mount a head. The new pod not only has the platform adapter, but the center column can be mounted horizontally (no need to remove the head to get down low).
The older model typically has a two part round center post that you can shorten (look for the parting line and setscrews) for low angle work. If that's not low enough, getting a spare center column and cutting it down is the best suggestion I have.