Why take the chance? Two options: spray the area with Roundup, let everything die back, cut it to the ground with a weed whacker and add your soil. Alternately, if you're ooked out about using a chemical control, cut everything to the ground first, then lay clear plastic over the ground for a few weeks to a couple months (depending on where you live) to cook off all the weeds and seeds. Either way, you'll want to add some compost to the added soil unless you're getting expensive, killer topsoil.

One other thought, just since you mentioned you're a novice. Is there a drainage reason for the plant beds being so high? Were the beds raised because the area floods in heavy rains? I have the double-edged sword of living two doors down from the grandson of a previous owner. There have been a couple of times where I considered changing something that seemed, on its face, to be stupid, only to find out that it was done forty years ago as a solution to another problem. Worth thinking about.

Thanks for your reply. I think I'll use round-up. About the drainage...the yard backs onto a lake, so I figured that I'd have to make a downward slope. The end of the yard currently sits about 3 feet above the lake (after a good rain).

That works. If you're right on the water, be 100% certain to mix the roundup according to directions and spray carefully. The inactive ingredients have been shown to have nasty effects on frogs and other aquatic critters.