The answer varies depending on your expansion scenario.
Detach an EVS disk online. For details, see Storage > Detaching an EVS Disk from a Running ECS in the Elastic Cloud Server User Guide.
Currently, SCSI disks can be attached to BMSs and used as data disks. You can detach a data disk either from a running or stopped BMS.
If a data disk is detached while its server is running, the server does not need a restart after the data disk expansion.