Iteration inside a Measure with ADDCOLUMNS & CALCULATE

Hi BI Users,

 Interesting question which may be lacking some conceptual understanding 🙄

I need to measure the Net Revenue Retention, we define it as the amount retained by a customer over a period of time withouth including upsales, so if a client purchased 100 on the previous period and purchased 150 in the current period we only consider the 100 as a retained amount. 

So my aproach to the problem is... add a column to a table with key = the DISTINCT CompanyID which will sum the amount purchsed in the last period and another column with the amount purchased in the current period to then use the logic above. Below is the code for the DISTINCT. (source code for some variables like "prevmindate" is not relevant)

Var mytable = 
    ADDCOLUMNS(
        DISTINCT(fac_cs_churn_renewables[account_id]),
        "Prev Period",
        CALCULATE(SUM(fac_cs_churn_renewables[amount_converted]),
            ALLEXCEPT(  fac_cs_churn_renewables,
                        fac_cs_churn_renewables[close_date],
                        fac_cs_churn_renewables[account_id],
                        fac_cs_churn_renewables[account_segment]
            ),
            fac_cs_churn_renewables[close_date] > prevmindate && fac_cs_churn_renewables[close_date] <= mindate
        ),
        "Curr Period",
        CALCULATE(SUM(fac_cs_churn_renewables[amount_converted]),  
            ALLEXCEPT(  fac_cs_churn_renewables,
                        fac_cs_churn_renewables[close_date],
                        fac_cs_churn_renewables[account_id],
                        fac_cs_churn_renewables[account_segment]
            ),
            fac_cs_churn_renewables[close_date] > mindate && fac_cs_churn_renewables[close_date] <= maxdate
        )
    )
Var newcol = 
ADDCOLUMNS(
    mytable,"final_amount",
    IF([Prev Period] >= [Curr Period] , [Curr Period] , [Prev Period])
)


RETURN
newcol

So if I run this code to create a table then it works, however, when I try to change the returning value to sum the "final amount"... example below

RETURN
SUM(test[Prev Period])

and plug it into a measure, instead of a new table, the code doesn't work properly.

Any help is highly apriciated, thanks in advance!!!