To put your property for sale and rent at the same time, you need to publish 2 separate listings (different conditions are considered as different listings even in cases involving the same property).
Only your first two listings are free on idealista. However, in this case the listing for sale and to let of the same property shall be treated as one for purposes of cost. This means that advertising your first two properties for sale and for rent will be free. Any following property listings will be charged depending on the area where they are and how long you want to advertise them for. If you add listings for sale and to let for the same property, the cheapest listing is free.
To publish your existing listing with a different type of operation, log into your account (top right of your screen) with your email and password. Once you’ve logged in, click on “Your listings” (in the green header). You will see the listings you have published. Find the listing that you want to change and click on “Publish your property as for sale / to rent, too – for free and in just one step“.