Why is the <dc:identifier> metadata element being changed or removed in the first place? Your original ePub indicates that it
has a unique-identifier in the <package> element, but then no unique-identifier is found in the metadata. A valid ePub
requires the dc:identifier, dc: language and dc:title elements. So your example ePub is just not valid right from the get-go (regardless if past versions of Sigil let that slide).
Are you asking for Sigil to maintain your ePub's invalid opf status upon first opening? Or are you asking for Sigil to be able to account for the myriad of ways an opf can be invalid and fix it in a non-destructive manner?
Quote:
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It opens with Sigil 0.34 without problems, actually it was created by Sigil 0.3.1 (as you can see from the metadata). Same happens when I want to change the BookId on any epub from UUID to ISBN within Sigil.
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If you want to switch to ISBN, then you need to change the scheme to ISBN in the opf file before saving and closing, just deleting the <dc:identifier> element and reopening won't work.
Change the <dc:identifier> element from:
Code:
<dc:identifier id="BookId" opf:scheme="UUID">urn:uuid:731aae73-ffff-469e-a5f6-b9c0ebca3360</dc:identifier>
To something like (with a valid ISBN, of course):
Code:
<dc:identifier id="BookId" opf:scheme="ISBN">0123456789</dc:identifier>
And save the ePub. All your existing metadata will be retained and Sigil won't rebuild your opf when you next open it.