so... well... for all that want to have new uuid generated bei opening "default.epub" and don't want to play with Sigil source there is this simple solution.
Linux (Ubuntu):
Rename "default.epub" into "default.zip"
Open "default.zip" with Archive Manager
Navigate to /OEBPS/content.opf and open it with text editor
Remove line <dc:identifier id="BookId" opf:scheme="UUID">urn:uuid: ... </dc:identifier>
Save
Archive Manager will ask if it should update file in archive "default.zip", confirm and quit
Rename "default.zip" to "default.epub".
Windows:
Something similar probably
Now every time you open "default.epub" Sigil will generate <dc:identifier> line with new uuid value.
On alas side:
Sigil will generate line: <!-- Your OPF file was broken so Sigil tried to rebuild it for you. --> in content.opf
toc.ncx is not immediatly updated.