No, you can not.
Yes, you can change it in profile section under plans tab. There you can select the desired plan and after the payment confirmation It will be reflected.
Yes you will be prompted to subscribe before your free trial ends.
When as the admin of a workspace you add an employee an invite link is sent to the added employee using which the individual can logon onto the workspace
A lead can be converted into a customer when you change the lead status to converted lead. This means that lead has now converted into a customer and their details can be found in customer section.
You can check under individual profiles of the employees for it and for overall you can track via a performance scoreboard widget available on dashboard