Shaving your dirt bike seat is a one-way ticket. You may be able to take more off, but you cannot put more back on. A shaved seat is a good alternative, or addition to, lowering your bike by using a lowering link or internal suspension work. I can pretty much guarantee that it will not look stock, and it will make your seat harder. If you have a KTM, the latter will not matter much as you are already sitting on a plank.
Shaving the seat provides a wider base that can be more comfortable than some stock width foams. It also provides an opportunity to roll the edges of the foam by sanding it for more custom fit, although you can do this without shaving the seat. The largest advantage to learning how to shave your dirt bike seat is being able to find the height that allows you to avoid tip-overs by being able to plant a foot, possibly even 2, on the ground or gain the additional room for movement you have been looking for without having to raise the pegs.
Having some additional clearance between you and the seat when things get rough is an added bonus. Learning how to shave your dirt bike seat is not for everyone and it can be costly if you mess up.
Before you proceed you may want to consider an aftermarket seat foam in either low, or soft. Durelle Racing, Guts Racing, and Enduro Engineering, and likely others, make some nice alternatives. I have personally owned seats from Durelle and EE. Both are high quality foams, but keep in mind that soft gel foams are heavier than stock seats but you simply cannot get more comfortable units.
If you are not interested in costly suspension work with the added penalties of lost travel and ground clearance, or, less expensive suspension options such as a lowering link and raising the forks in the clamps which still suffer the same lost ground clearance and can have a profound impact on handling, shaving your seat or a lower profile aftermarket seat are the options to be considering.
If you decide to learn how to shave your dirt bike seat there could be known and unknown costs beyond time, and there are no assurances that you will like a lowered seat. If you are working with an older foam, you may find it deteriorates or has de-laminated from the pan and needs to be replaced. While unlikely, you could uncover or potentially crack a seat pan.
Many aftermarket seat covers, especially the ones that come with the aftermarket seat foams, are not all they are cracked up to be and riders decide they need to purchase something different. If you are looking for a new cover regardless of the reason, you should be safe with just about any SDG or Guts season. Then you start getting picky and realize where grip matters or causes you grief, if ribs matter to you or if you can even tell they are there, etc.
If you are planning on buying an aftermarket lower seat but are just not sure if the lower height will be a benefit, go ahead and whack your stock unit down and see how if feels. If you manage to salvage the stock cover long enough to at least try out the new height you will be out nothing but time.
Should you decide to take the plunge and learn how to shave your dirt bike seat, take your time. It should go without saying, but you can take more foam off but you cannot put more foam on. Okay, technically you could add foam, but we will stay away from that one for this article.