Whether the threadjacker changes the subject line to reflect the change in subject or retains the original email subject, to take over a thread can be regarded as threadjacking in either case.
Threadjacking can be both deliberate and, more commonly, inadvertent. Threadjacking can mess with email program and services organizing messages by conversation. A new topic in a threadjacked thread can be difficult to spot.
How to Avoid Threadjacking
To avoid threadjacking, create a new message in your email program or service to embark on a new conversation topic instead of replying to an existing email from the desired sender or list.