There are a couple of ways that this can be done. The easiest would probably be by using either an acf/load_field filter (https://www.advancedcustomfields.com/resources/acfload_field/), an acf/prepare_field filter (https://www.advancedcustomfields.com/resources/acfprepare_field/) or an acf/load_value filter (https://www.advancedcustomfields.com/resources/acfload_value/). Any one of these can be used to alter the default value of the field when the page is loaded and or the field is shown. acf/load_field and acf/prepare_field will let you alter any field settings.
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