The field types for custom properties in HubSpot can be changed. It is not possible to change the field type of a default HubSpot property. See this article for instructions to change a single line text property to a drop-down select property.
Please note: If you convert a single line text field to a drop-down select, existing values that contacts already have will not appear as property options. However, contacts who had values for the property in question will keep the previous values they had.