If you cannot resize user partition, then when CE boots from 8GB card make a backup using the option in Coreelec Configuration -> System. Transfer this file from storage/backup to some USB stick.
Make a fresh install to 32GB uSD card, which will properly resize user partition, and after it finishes booting restore (again from Coreelec Configuration) your safely stored backup file.
Everything will be as before with full size of user partition…