The subsidiary is decided during the registration and it is based on the country of residence you have selected. It will then populate the subsidiary field (its a drop down) based on the country of residence you have selected. Some countries might be able to select another subsidiary.
This is because the country does not have a local subsidiary (office) but due to factors such as language of the country, multiple subsidiaries can accommodate you as a customer. A good example is Switzerland. Since locals may speak French or German, we allow customers living in Switzerland to choose to register with OVHcloud France or OVHcloud Germany.
Changing subsidiaries is not possible due to several factors such as a possible pricing, currency, terms and conditions and policy differences.
I would recommend you to reach out to support to find out what would be the best option in your case to do. You might need to create new account(s) and then you will be able to change them.