As opposed to both the expense, and uncertainty of getting a perfectly seamless-appearing union between two Plastic sheets, correcting the Drywall issues and repainting is a vastly superior choice.
Unless....your wall is a complete disaster. Uneven studs, poorly done Tape joints, deep nicks, cracks, gouges....all present needs and skills few DIY'ers posses. Even then, if the Screen Real Estate is epic, overlaying and finishing Drywall is still a "most economic" choice whose final finish is at least dependent upon you/others efforts....not a "mfg".
Minor blemishes that a High Contrast paint amplifies....? You simply "erase" them, blot 'em out, smooth, prime, and paint again. Be assured, if the Framing is true, and it's simply a Texture or paint blemish issue, correction is a better solution that trying to create an ideal surface from "two" ideal surfaces.
Of course, the latter "could be done", but one way of 'doing' it requires a purposefully made, beveled edge seam that is filled and float sanded into a light skim of Drywall compound that covers the entire expanse of the Sintra sheets. Everything is sanded equally. If your gonna have to do all that....you might as well be doing it with the original Dry wall.
And...there are other inexpensive choices. Large 17 mil sheets of vinyl, 10' x 12". IMO a better substrate to start out with than BOC, and it's no more expensive "than".
DIY can take you down some very different paths. The most used applications and methods are usually the most successful. Some simply require a degree of "opt in" effort that is higher than others, but very few who have gone down such paths will dispute the worthiness of such a choice.
atl999, if you hit upon a specific choice of action "you" feel comfortable with, you can count on all the advisory help you'll need, and you're end results will be better for it.
But if your wall is basically sound, no other method I know of will get you to your goal more quickly, effectively...and most assuredly less expensively than getting the wall in shape and repainting.
If the Dry wall isn't hanging on just a few loose screws/nails, and there is no dry-rot issues, a wall coated with decent Skim Mud pre-prep / finish coat of SF will hold up longer than you'll have use for it.
Don't worry...chose this route, and you'll get the necessary help too.