1) We released 4.5 update for the WooPress theme that is compatible with the WC 3.5. So, we would recommend you to update parent theme.
2) Everything depends on changes that were made in child theme files. If you used the old WooCommerce code and old WC functions that were changed/removed after the 3.5 update, you will need to check the files of our parent theme and add all the necessary changes to the child theme files that you re-wrote.
It does not mean that you need to create child theme from scratch but probably you need to fix some moments to be up to date.