Roof rack cross bar length adjustment, W210 S210, wagon

[Shortsguy1]

Hello-
I am the newish owner of a 2003 W210 (S210) wagon. I recently found a pair of MB roof rack cross bars at an auto salvage yard and would like to use them on my car. Sorry if this is a dumb question, but I am trying to figure out the correct way to adjust the length of the cross bar. I have searched and searched for this question, but was unable to find the exact same issue. Attached is a picture of a cross bar on my car (see first pic, and please excuse the drippy appearance which is Kroil which I sprayed on the thing).

At the length I found them, it has to sit right above the sunroof, which makes a lot of noise when I have the roof open. I would like to move the front cross bar further back, if at all possible. Since the side rails get closer together, this will require slightly shortening the cross bar. They have a slot and appear to adjust in length, but I don't want to break anything. Mine certainly don't slide easily but I haven't bashed the thing too hard yet. Please see the second picture (which I stole from Ebay; thank you random ebay user) for the underside where you can see a slot and maybe a pin in that slot.

Any thoughts on changing their length? Do I just need to be more brave with use of force? Thanks for your help.