Hi Janmel,
You will get an email before you are scheduled to upgrade your service. For more information about the service upgrade, you can check out the Office 365 service upgrade center in the link below.
Before that, if you want to use Office 2013, you can try a trail for the new Office 365 and install Office 2013 from the Portal. Your current account of the pre-upgrade tenant with Office Professional Plus license can be used to activate the Office 2013 applications installed from the trail tenant.
Also, I’d like to include the following links for your reference.
Office 365 Trail link: office.microsoft.com/.../compare-office-365-for-business-plans-FX102918419.aspx
Office 365 Service Upgrade Center: community.office365.com/.../office-365-service-upgrade-center-for-small-business.aspx
Thanks,
Nancy Xie