If you are reading anything that makes you think that we bloggers can install plugins, custom themes or third party themes on sites hosted by wordpress.COM, then you are mistaken/misinformed. We have never had that ability. You are confusing wordpress.COM hosted blogs and wordpress.ORG installs on paid hosting.
Blogger installed plugin capability does not exist on WordPress.COM blogs and there is no upgrade you can purchase here that changes that reality. Permalink structure cannot be changed on sites hosted by wordpress.com