You can remove them but there is a good chance they will get bent. You have to use a tool that helps you gently lift them out. They are soft aluminum. Only a slight angle at a time. They have a slight bulb at the bottom of where they are inserted into the rubber gasket. You have to lift this bulb section out of the rubber holding it. You do just a little at a time. NOT all the way out all at once.
If your windshield gasket is old and hard, just cut it out as suggested and replace it.
A good quick test of the gasket is to lift a flap. If it snaps back it is probably good. If it falls back slowly (anything other than snapping back) it is aged and will eventually start leaking.
And installing the window: Definitely put the gasket around the window, install the metal frame, lube with something like spray silicone both to insert the molding into the gasket and to help the gasket's rubber flap slide over the corresponding metal channel of the body of the car. Use a nylon rope, greater than 1/4", better 3/8" which is inserted all around the the deep channel in the rubber gasket and have it overlap at the bottom.
Insert this assembly into the car (from the outside) with the top into the metal channel first. Make sure the sides are aligned, push the top against and into the channel at the top first
Have a friend help by applying even and light to moderate pressure just to hold the glass against the metal body channel at the bottom. Nothing intense, just holding it down. Start pulling the rope and have the assistant *gently* slap the glass as you pull, going all the way around the glass. In this process you can adjust a little bit. If you are way off, start over.
To help the rubber flap of the gasket get over the metal channel of the body, you can use a *very* soft headed, rounded end, metal or plastic tool. You can buy cheap auto glass tools just about anywhere.
I have done four windshields/back glass this way and each have worked out great. An old glassman showed me all this years ago.
Hopefully this was clear.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)