If I upgrade to Plus in order to view/print a previous year's tax return, is it a one time fee, or will I be charged yearly? Will I be charged to print previous returns?

Once you upgrade and purchase the 2017 Plus edition you will have access to all the online prior year tax returns through October 2019.  Once you have access to the prior year returns there is no charge to download a PDF for any of the online returns for printing.