If I upgrade and make changes will they still be available for me if I downgrade the CRM?

If I upgrade and make changes will they still be available for me if I downgrade the CRM?

I would like to add or to modify some fields. I am using a free account. What if I upgrade the account for let´s say 3 months and then I downgrade the account again after that period of time. Will the changes I made still be usefull? Or they will get lost with the downgrade?
Thanks