How to install Plesk over already installed ProFTPD service?
Currently Plesk do not have such functionality, feature request #1777149 already created.
Feature request will be reviewed by the developers and may be considered for implementation in one of the future product releases. Please use the id provided for your reference.
http://kb.plesk.com/subscriptions/index.html - knowledge base, each product update comes with corresponding KB article;
Additionally, you can use RSS feeds at http://kb.plesk.com/rss/index.html