Welcome to the community!
It's not really possible to answer your question without knowing more details of your plan. However, if you are financing your phone with the payment program, charges for changing plans depends on your current plan and desired plan:
~taken from link above.
If you are not sure whether those plans are of the same category, you might consider contacting customer service. Alternatively, you might consider sending @FidoSolutions a PM. Once they verify some information, they'll also be able to access your account.
Hope this helps 😀