As the 8.1 is an OEM (MSDN) the bios key will kick it BEFORE activating so you should be fine... I know 8 and 8.1 msdn get along well in this way but just unless Microsoft takes new steps to stop people doing this with 10 is still up in the air.
1. format some blank (unallocated) space before getting the 10 upgrade which does go over the top of your 8.1.1
2. once the 10 is fully installed, boot to a disc/ usb and install 8.1 onto the unallocated space then set which one you want as the default.