The FAQ says that California and New York datacenter users will need to back up their data.
If you are in the California or New York datacenter, … You[r] data will be reset during the migration, so make sure to backup your hard drive prior (you won't be able to do it during the migration).
Please note: migrations in the California & New York datacenters require a hard drive reset because your account is transfered to a new datacenter.
However, the FAQ also states additional storage will remain intact during migration.
Your additional storage remains intact as an add-on to your base subscription.
Will California and New York datacenter migrations reset customers’ main drives but preserve their additional storage?
Best answer by ZedOudView original