After you install Pro it might be best to auto-disable the free version since they cannot run at the same time. A lot of Pro plugins build upon free and therefore clients regularly keep both active.
Alternatively show an admin notice to inform the user 🙂
See list below (taken from Query Monitor).
Warning Constant AWCDP_TOKEN already defined
Warning Constant AWCDP_VERSION already defined
Warning Constant AWCDP_FILE already defined
Warning Constant AWCDP_PLUGIN_NAME already defined
Warning Constant AWCDP_TEXT_DOMAIN already defined
Warning Constant AWCDP_STORE_URL already defined
Warning Constant AWCDP_POST_TYPE already defined
Warning Constant AWCDP_DEPOSITS_META_KEY already defined
Warning Constant AWCDP_DEPOSITS_TYPE already defined
Warning Constant AWCDP_DEPOSITS_AMOUNT already defined
Warning Constant AWCDP_DEPOSITS_FORCE already defined
Warning Constant AWCDP_PLUGIN_PATH already defined