I tend to not run the installer or involve a USB stick. typically this means download the ISO, su to root. mkdir /data/mnt, where you'll mount the rootFS of your existing android. then i'll make two temporary dirs called A and B. issue a mount -o loop /sdcard/Download/Android9.0.0-a3.iso A. then mkdir a9, and cp A/{*.img,kernel} a9. then mount A/system.sfs B. then if there's a system.img in there you need to mount that in C. then cp -av C/ a9/system and mkdir a9/data. now mkdir boot, and mount /dev/sda1 boot and fish around and find the grub.cfg and copy/paste the a7 entry and change it to the /a9 path and boot and hope it works. mostly worked here on ASUS T300 modulo the usual bluetooth and instability issues that go along w/ alpha builds. good luck