Making changes to woocommerce order before save

A popular method is to hook to the order status.

You have several status names to choose from:

1. completed
2. processing
3. cancelled
4. on-hold

You would use it like this:

add_action('woocommerce_order_status_processing'

In addition you can intercept a certain change like this:

add_action('woocommerce_order_status_on-hold_to_completed'

Finally you can intercept any change that takes place like this:

add_action('woocommerce_order_status_changed'