How to alias column into another operation

I want to use the Elias column startdate into anthor operation to calculate

CASE WHEN GETDATE() > detail.PeriodEndDate  THEN  GETDATE() 
                 ELSE detail.PeriodEndDate
        END as StartDate,

          CASE
                 WHEN usrPlan.PackagePlanId = 1  THEN  DATEADD(month, 1,  StartDate)
                 ELSE  DATEADD(YEAR, 1,  StartDate)
        END as EndDate,

Answer

Use CROSS APPLY to define the alias in the FROM clause. Then it is available throughout the query:

SELECT . . .,
       v.StartDate,
       (CASE WHEN up.PackagePlanId = 1 THEN  DATEADD(month, 1, v.StartDate)
             ELSE  DATEADD(YEAR, 1,  v.StartDate)
        END) as EndDate,
FROM usrPlan up . . .
     detail d . . . 
     CROSS APPLY
     (VALUES (CASE WHEN GETDATE() > d.PeriodEndDate THEN  GETDATE() 
                   ELSE d.PeriodEndDate
              END)
      ) v(StartDate)

Note that I added table aliases so the query is easier to write and read.