You can keep your old installations and install the Buster iso to a new partition. But you will probably have to resize one of your existing partitions to make room for it (assuming you didn't leave empty unpartitioned space on your HD.) This isn't hard from the installation menus as you are installing.
Generally Windows partitions don't like to be resized, and you end up having to fix the Windows install after resizing it. (Something I'm sure you had to deal with when you installed Mint before.)
First check that you have enough free space in the partition you plan to resize. Then boot the install iso. When you get to the partitioning screen select manual. Then select the partition you want to resize. Put in the new size. Then select the newly created free space and partition it, and set as installable. You can share the swap partition you used with Mint. Then install.
It's been a while since I've done this, hopefully I didn't skip too many steps or get them wrong to keep you from figuring it out. You can also Google more specific instructions and watch a tutorial vid or two. Most any generic Debian install instructions should cover this.
