Switching to larger backup drives

I have a Windows 2012 R2 Essentials server with a 1 TB mirrored drive.  I also have four 1 TB backup drives that I rotate, with one off-site.

EDIT: I am using Windows Server Backup

However, my backup drives are almost full and I need to use larger drives.  So I bought three 3 TB drives.

My question is: if I do backup setup again with just the three 3TB drives, do I lose access to all the older data that was on the 1TB drives?  To guarantee backup integrity, should I instead do setup will all seven drives attached?

Thanks in advance for any answers.

- Grant