Generally speaking, you should be able to change the durations in listings you have already posted provided it is still allowable based on the revision restrictions described on the following page of eBay's Help index:
In order to revise the duration of an active listing through Auctiva, you would want to check the box next to the item on your Active Listings page, click the “Bulk Revise” button, select “Duration” from the “Select Field to Edit” drop down menu and then proceed as directed.
However, eBay will not allow to change change the duration of Good 'Til Cancelled listings nor change the duration of a current listing to Good 'Til Cancelled, so it will not be possible to make the specific revision you have described.
In this particular case, I believe your best options would be end the item that was posted with the incorrect duration and then post it anew with the Good 'Til Cancelled duration or revise the duration to 30 days and then post it anew as a Good 'Til Cancelled if it does not sell during that 30 days.
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