I suggest you find needed php_handler_id with
# plesk bin php_handler --list
and then update domains with this php_handler_id with something like:
# for i in `mysql -uadmin -p\`cat /etc/psa/.psa.shadow\` psa -Ns -e "select name from domains"`; do /usr/local/psa/bin/subscription -u $i -php_handler_id plesk-php71-fastcgi; done
Of course, you can modify this script with more precise selection of necessary domains.