Hi Janmal,
You can add the additional 100 users from Office 365 purchase site. However, you are unable to convert the subscription to Office 365 Enterprise E1 or E3 within the current tenant (as these two kinds of subscription are in different service families). If you convert the subscriptions manually, you need to back up the data, cancel the current subscription (you will not get a refund from Office 365 with the repaid subscription), and sign up for a new plan.
Thanks,
April Zhou