EF Core Transactions, select a value before updating; Moving a HierarchyId Node to a different parent

Question

The suggested SQL code by Microsoft to move a HierarchyId node is:

CREATE PROCEDURE MoveOrg(@oldMgr nvarchar(256), @newMgr nvarchar(256) )  
AS  
BEGIN  
DECLARE @nold hierarchyid, @nnew hierarchyid  
SELECT @nold = OrgNode FROM HumanResources.EmployeeDemo WHERE LoginID = @oldMgr ;  
  
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE  
BEGIN TRANSACTION  
SELECT @nnew = OrgNode FROM HumanResources.EmployeeDemo WHERE LoginID = @newMgr ;  
  
SELECT @nnew = @nnew.GetDescendant(max(OrgNode), NULL)   
FROM HumanResources.EmployeeDemo WHERE OrgNode.GetAncestor(1)=@nnew ;  
  
UPDATE HumanResources.EmployeeDemo    
SET OrgNode = OrgNode.GetReparentedValue(@nold, @nnew)  
WHERE OrgNode.IsDescendantOf(@nold) = 1 ;  
  
COMMIT TRANSACTION;
END ;  
GO

I know I can use the stored procedure directly with EF Core, but prefer to have it as a normal C# code transaction.

My general question is how would I do the selects within the transaction to update values with EF Core? Or even better, what would be the equivalent code in C#?