You can use the pmpro_set_express_checkout_nvpstr filter here (https://github.com/strangerstudios/paid-memberships-pro/blob/dev/classes/gateways/class.pmprogateway_paypalexpress.php#L96) to change that instead of changing the core plugin directly.
Something like this:
https://gist.github.com/strangerstudios/377c3a45b5dbecb9ef87