Hi, icearch.
If you're already using Sigil, you can directly replace the image (without deleting it first) with a new one of the same name.
In the following dialog, clicking "Yes" will keep the "Cover Image" property in the new image.
[P.S. If you're in a real hurry or using a device without Sigil, opening the epub file with a zip-compressor and replacing the image will achieve the same effect… although it won't pass validation (due to the relocated 'mimetype'), most apps will open it without problems. For a quick personal test, it works.

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