To set a billing plan, use the command account/update_plan:

svc=account/update_plan&params={"itemId":<long>,
				"plan":<text>}

 Attention!
To assign a billing plan to an account, the current user must have the right to manage this account (see Access flags: Resources). Besides, the current user must be a direct or indirect parent of the account.

{}	/* empty object if execution successful, if not - error code */