Hi there,
Looking at https://docs.woocommerce.com/wc-apidocs/hook-docs.html and searching for filters that relate to downloads, I am not seeing any that look like they might be useful for manipulating the file name.
For assistance with customization or development with your site, we recommend that you seek help from:
* A local web developer
* Codeable.io (https://woocommerce.com/codeable/) who offer free estimates
* WooExperts (https://www.woocommerce.com/experts/)
* Stackexchange (https://wordpress.stackexchange.com/help/on-topic)
As you are comfortable with coding yourself and have questions, I would recommend that you join the WooCommerce Slack Community (https://woocommerceslack.herokuapp.com), or the Advanced WooCommerce Facebook group (https://www.facebook.com/groups/advanced.woocommerce).?
I hope that helps you to figure it out.
Feel free to get back to us if you have further questions.