The new version of MediaWiki will be on test wikis and MediaWiki.org from 16 March. It will be on non-Wikipedia wikis and some Wikipedias from 17 March. It will be on all wikis from 18 March (calendar).
You can add a newline or carriage return character to a custom signature if you use a template. There is a proposal to not allow them in the future. This is because they can cause formatting problems.
You will be able to read but not edit 12 wikis for a short period of time on 23 March at 06:00 (UTC). This could take 30 minutes but will probably be much faster.
You can use Quarry for SQL queries to the Wiki Replicas. Cross-database JOINS will no longer work from 23 March. There will be a new field to specify the database to connect to. If you think this affects you and you need help you can post on Phabricator or on Wikitech. PAWS and other ways to do SQL queries to the Wiki Replicas will be affected later.