You can't do a dirty flash from Cr8 => 7, a clean flash of the rom will be required in that case.
You need to backup your user app with swift back up, your contact on a vcf file and the photo or any important data manualy into a safe place (on the cloud or another device).
Then remove your lock pattern (for remove encryption), and place a SDcard with CR11 .zip, gapps, magisk. Reboot on recovery, check if your SDcard is recognized then wipe system, data partition and flash Cr7.
If Cr7 was stuck at the boot logo or if your recovery can't decript the files after set a lock pattern, you will need to format data.
I use microG so I don't know which is the best gapps version (Nikgapps should work).
While cr 7 booted successfully, you can import all your data.