What you can do is get an appropriate nut that will screw on to the 3/8's adapter. Go to an auto store and buy some red loctite. Put a little on the threads and screw on the nut. Wait a few hours. Then use a wrench to take it off. Note: This will be the end of the adapter and the nut. So muake sure it is actually a screw on adapter you've got going on.
Sadly, Carson's post might be on the right track (though someone beat him too it)...now that I look at it closely, there's some red around the top of the 1/4 post...I think the former owner might have put some red loctite over it so it's inside the 3/8 adaptor?
Thanks Dan and Jim, but it definitely doesn't push down, it looks threaded to the 1/4 post?
Anyone have a cheap RRS style clamp that will fit a 3/8 post?
an easy way to kill red loc-tite is to heat it up...
if you have a soldering iron... heat it up and apply it to the 3/8" adaptor... it will heat up in about 30-45 seconds and then you can use a crecent wrench or a standard pair of pliars to "break" the seal. If you don't have a soldering iron, find a larger nail (bigger than a brad used to hang a picture, a 10d nail should do the trick... That's a 10 penny nail not the camera...) and heat that up on the stove while holding it with your pliars... once the nail gets hot.. take it and touch it to your 3/8 adapter and it should heat it up enough to losen it....
Other trick is dry ice, find an ice-cream shop... most will have it.
I use red loc-tite on my scopes and sights... and this is how I remove them, this trick also happens to work on plumber's epoxy
Can you slide the grip off the monopod? The end with the stud is probably reversible. I have a Bogen monopod with a different head but it has both 1/4" and 3/8", you just turn it around for the other.
I bet something threads into the top of the monopod and the grip just slides down over it. Stick something up under the grip (screwdriver, etc.) and squirt some hairspray in there, rotate it and it'll slide right off.
I bet you can then unthread the stud, flip it around.
Any other ideas?
Yeah, you could use a die and cut it down to 1/4". BTW I'm in Portland, too, and probably have the right die downstairs... hmm...
most of you should stick to taking pictures rather then destroying gear . have you tried calling bogen to get a definitive answer? their service personal can be helpfull. the model number is etched or on a sticker on the top tube. because at the rate this is going it may end up there anyway.
Another option is an adapter 3/8 to 1/4. I use this when friends want to use my tripod. Gives you the flexibility to change your mind. And no destructive tendencies
I have two monopods, one has an adapter that threads over and one is pushdown. You probably want to try pushing it down first. 
BTW I'm in Portland, too, and probably have the right die downstairs... hmm...
. have you tried calling bogen to get a definitive answer? their service personal can be helpfull. the model number is etched or on a sticker on the top tube. because at the rate this is going it may end up there anyway.