Best way to convert adjustable shelves?

The sides are just as thick...1-3/8." The dowle or screw method should work. The bottom is already dowled and sits flat on the floor. There is a leveling issue there with the wood floor so I may have to figure out a way to deal with that...so the cabinet doesn't rock, is level, and does not damage the floor.

The shelves are 21" x 16" so they're perfect for my components.

So your inside cabinet width is roughly 21.375 inches. How much width do you require? You can either laminate .05" or .75" plywood to each inside cabinet wall, allowing for a precise dado for each shelf. Using .05” plywood will leave you a rough width of 20.075'; using .075” plywood will leave you roughly 19.875".

You can purchase birch plywood (which nicely accepts stain and can be matched very closely to maple), the width of which should be ripped to the same spec as the inside depth of the cabinet walls -the length of which should be cut to approximately the same spec as the inside height of the cabinet. Now, figure shelving positions and mark the inside of the cabinet (precisely). Then cut your pre-ripped plywood to the necessary lengths to fit above and below each shelve (leaving some room for the glue/cement). Use quality wood glue and screws to secure the plywood to the inside cabinet walls. Drill/countersink screw holes in the plywood so that you secure the screws from the inside of the cabinet (which can be covered with dowels – best, or with wood putty – won’t look professional). Insert your shelves by sliding them in place via the rear of the open cabinet. I recommend using an acoustically designed speaker cement or silicone to “glue” the shelves in place.

2chnlben - Those are great suggestions! Virtually eliminates any weight bearing concerns and maximizes the wood coupling for the entire cabinet...looks good too. Thanks.