Ah ok. I understand now. I wasn't picking up on what you were saying before.
If with the soft launch you are already putting the "new" version of the site on their intended final URLs then yes, you can let the engines start crawling those URLs. For each new URL you let the search engines crawl make sure to 301 its corresponding old URL (the old site) to the new version to minimize any duplicate content issues.
If for whatever reason you can't quite 301 the old URLs yet (like if you still need instant access to reroute traffic back to them) you could try using rel=canonical on the old pages and point them to their new counter part only if the main content on each of the pages is almost exactly the same. You don't want Google to think you're manipulating them with rel=canonical.