Originally Posted by aka123

As I just added in the previous post; if the sitemap is automatic you don't have to do anything if it updates properly. Though for example many sitemap plugins create a new sitemap if there isn't any, so deleting the old one is just fine.

If you mean submitting sitemap into google, you can submit it as soon as you have something to submit (aka site up).

Nothing else comes into my mind. This is quite simple case. Well.. one thing came into my mind. If you have similar site than you are now giving up (the old niche), you can of course redirect the relevant links to some another relevant site. But I don't know the exact effects of this as usually when thing like this is done, the old domain is not used again for something else. At least visitor wise it would be more user friendly as they would land on relevant page.