Are you talking about authorizing a computer in iTunes?
It is doen form the Store menu in iTunes. It has to be done from the computer that was authorized.
If that computer is no longer used your only option is to reauthorize all computers, then add them again. You do this in your iTunes account settings, accessed by clicking on your Apple ID at the top right of iTunes. You can only de-authorize if you've used all 5 of your allotment, and you are only allowed to use the de-authorize once in 12 months (I think). I you have't reached the limit, don't worry about it for now. If you do use the feature, remember that your other computers will need to be authorized again, from the Store menu in iTunes.
You iDevices do not use up any of these authorizations.
If you are talking about the pairing of your iDevices to a specific computer, you don't need to worry about taking them off the old one but you need to be careful about how you connect them to the new one. Here are the best instructions for that I've managed to put together. I keep looking for better ones, so if you find some let us know.
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Here is Apple's recommendations on how to transfer all your stuff to a new iPhone. With a few modifications it works very well for moving your iPad to a new computer. Wish I could find one as well written for the actual task.
How to back up your data and set up as a new device
Start by going into iTunes Preferences (before connecting the iPad) and turning off auto-sync. It is under the Devices tab. This will prevent any unwanted syncing before you are ready.
Do only the parts that apply to your information/situation (i.e.; if you are syncing contacts with Google, don't do the Personal Information Manager part. Stop when you get to the Restore step.
It would be wise, after the Transfer Purchases option to go through iTunes and make sure everything (media, app, book) got transferred. If not you may have a few more steps to insure all your stuff gets safely backed up on the computer.
NOTE: If you have picture albums in the Photos app, and they are not backed up somewhere other than the iPad, you need to save them first before doing anything else. The first sync will remove them. The PhotoSync app works well for this, and you can use it before starting any of the other procedures.
The quality of the pictures recovered this way will probably not match the originals. iTunes optimizes photos for viewing on the iPad when it syncs them.
