Yes, you'd have to scrape off some of the existing textured finish to do the tape job. The good news is since you have a textured ceiling, if your tape job isn't that good, the textured finish should cover up any mistakes.
The taping has a few purposes. Since most drywall is either beveled or tapered, by putting mud in the joint and taping, you keep the sheets 'together'. Also, the tape job makes the wall/ceiling look 'seamless' or as 1 sheet. The mud will most likely crack over time if you don't use the tape. The tape keeps the mud from 'moving' too.
As for doing the texture part of the job, you'll have to use whatever you can to duplicate the texture. Or, you can search the WEB for sites that might have a tool for your type of ceiling. Here's one site: Drywall Textures. I think you can e-mail them with a description of your ceiling. AT worst, take a good picture of your textured ceiling from a 45 degree angle (so the flash doesn't cloud the picture), have someone scan it in, and e-mail it to Drywall Textures or anyone in the business to see if they can tell you what you need to do. Other than that, I haven't much else to add. BUT, do ask; someone else may.