Bilge has made a very important point, which I will add a smidge to.
You can/should, must if the finish is gloss, sand the old finish a bit for cleanliness and to provide tooth for the new finish. Before you begin you ought to decide whether you're preserving the old finish or removing the old finish to expose new, fresh wood. If you're preserving the old finish then you use fine sandpaper, 220 grit is pretty normal, or fine steel wool. The original manufacturer's finish normally will have been sprayed on and is pretty thin; sand just enough to make it uniformly dull and clean. There's just a little latitude here, it's easy to sand through the finish to the wood. If you sand through, you'll learn to leave the deeper dings...it's called 'character', 'patina'. If you sand through, there's more care to be taken in staining and refinishing since the raw wood will take finish differently than the sanded adjacent finish.
If you decide to remove the old finish, the veneers on production plywoods are quite thin. They used to be, say until the '50s or '60s, 1/32" thick; now they are routinely 1/48" thick. That's not many strokes of sanding and precious little if you're using a scraper and heat gun. It can be done, I've done it, but proceed, oh, so gently. A palm sander with fresh 220 grit paper is remarkably fast sanding through 1/48" veneer!
Entertainingly, some production plywood is made with veneers that were sewn together into sheets before being put in the plywood manufacturing press with the core veneers. When the glue had cured, the new plywood was lightly sanded to remove the threads from the surface and the stubs of thread did not show. When you refinish plywood like this, you may sand through enough to see the threads that remain in the glueline zig-zagging along the joints. Very discouraging! (We rejected purchasing a sailboat because of this... and it's 7' draft.)