An ownership change is not a single immigration event. Depending on its legal effect, it can change the identity of the person controlling the licensed organisation, move sponsored workers to another employer or trigger a requirement for a replacement licence. The fact that a report was omitted is only the first part of the analysis.
Where the error is discovered months after completion, a sponsor should resist the temptation to file an abbreviated notification and hope that the issue is treated as historic administration. The Home Office will be interested in what changed, when it changed, which workers were affected, who knew and whether the present compliance arrangements can be trusted.