Question about employment contract changes in Japan

I’m looking for general insight from others working in Japan.

Due to a close family member becoming seriously ill overseas, I contacted HR to discuss the possibility of changing my work arrangement in the future (for example, whether part-time might be an option if I needed to travel more). This was meant as an initial conversation only — I did not submit a formal request, sign any documents, or give written consent to change my contract.

After the family member passed away, I contacted the company again to let them know I would be returning to Japan soon. At that point, I was informed that my contract had already been changed, a replacement had been finalized, and that I would no longer be able to continue in my original role or location as of a future date.

Given that no new contract was signed and no official request was made, is a company legally allowed to:

• Change an employee’s contract type unilaterally

• Proceed with hiring a replacement

• Inform the employee after the fact that their original position will no longer be available

Is explicit consent and a signed contract normally required under Japanese labor law in situations like this?

Any general experiences or legal insight would be appreciated.

by satellite_station