In Exchange Online and so far I know, it is not possible to separate threads based on changed subject lines as it uses a conversation threading feature that groups all messages with the same subject line together, regardless of whether the subject line has been changed in subsequent replies.
However, you may be able to achieve some level of separation by using categories or folders to organize your emails. For example, you could assign different categories to emails based on their subject line or content, or move emails to different folders based on their content.
Another option is to use third-party add-ins or tools that allow for more granular control over email organization and threading. There are several such tools available that can integrate with Exchange Online and provide additional features and functionality.