Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Addition of the Exchange Plan 2 will not do any change to size your current mailbox.
You need to use the Exchange Online auto-expanding archiving feature to enable additional storage space for archive mailboxes. When auto-expanding archiving is turned on, additional storage space is automatically added to a user's archive mailbox until it reaches the storage limit of 1.5 TB. You can turn on auto-expanding archiving for everyone in your organization or just for specific users. For more information about auto-expanding archiving, see Learn about auto-expanding archiving.
for your reference: Enable auto-expanding archiving