But the Named Credential hasn't changed between current and new version. So this Known Issue doesn't apply: https://success.salesforce.com/issues_view?id=a1p3A000000mCFtQAM&title=cannot-modify-managed-component-attempting-to-upgrade-managed-package-containing-named-credentials
Did there anything change with Spring'19 that could create this problem?
Note: I also created Support Case 22042201 for this.