[Galaxy S II Android Development] the.gangster:
[MENTION=8002343]neuweiler[/MENTION]

One of the specialties of the experimental build is, that it's otasigcheck accepts the releasekey of the "old" CM13 and unofficial CM14.1-builds

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

as well as the new release-keys of LineageOS officials

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

to make the move to the new signatures.

If you really want to disable this kind of safety-check for compatibilty, you could e.g. remove the check in the updater-script (lines 10-18). (not meant as a recommendation, but to work around the failure. It's your decision). Officially, you need to wipe data, as the check evaluates /data/system/packages.xml.