Currently, as per learn.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/lifecycle-services/configure-service-updates, it is not possible to change the target sandbox for service updates once the rollout has started:

After the settings are saved, you can change them at any time. However, if there is an ongoing rollout, the new settings won't be used to update the existing rollout timings. Instead, they'll start to be used in the next rollout. An ongoing rollout is defined by the 14-day period between the date when the email notification about the update of the sandbox environment is sent and the date when the production environment is updated.

I kindly request that this logic is changed, to allow administrator to change the sandbox once the rollout has started, at least for the first 2-3 days, to be able to amend the target sandbox in case it was selected