1. 7 days
2. Yes, just follow the ADSL tracker (connections settings link on left) or keep an eye on your email and via the contact us ticket system (also on the left).
3. As soon as your migration has completed and you have connected via plusnet. Just click on the upgrade account icon under connection settings and follow the pages to select what you want. It will cost £14.99 to do so (unless you migrated to an annual contract and you selected to upgrade to a monthly contract when it will cost £24.99).