There's no need to have broken links. 301 redirects are your friend!
Do some research now to list all your existing URLs, so they can be properly redirected to the equivalent new URL when the time comes.
Also do some research to identify who is presently linking to your site. In some cases it would make sense to ask the referring site to update the URL. In other cases it would make sense to leave the link alone and let the redirects take clickthroughs to the new location.