How to change column letter in formula while dragging down?

As the title says, I wanted to changed the column letter while dragging down this formula:

=IF($P$1=2019,SUMIFS(MOOE!$BM$4:$BM$1000,MOOE!$A$4:$A$1000,">12/31/2018",MOOE!$A$4:$A$1000,"<02/01/2019",MOOE!$I$4:$I$1000,"MOOE19"),IF($P$1=2020,SUMIFS(MOOE!$BM$4:$BM$1000,MOOE!$A$4:$A$1000,">12/31/2019",MOOE!$A$4:$A$1000,"<02/01/2020",MOOE!$I$4:$I$1000,"MOOE20")))

to:

=IF($P$1=2019,SUMIFS(MOOE!$BN$4:$BN$1000,MOOE!$A$4:$A$1000,">12/31/2018",MOOE!$A$4:$A$1000,"<02/01/2019",MOOE!$I$4:$I$1000,"MOOE19"),IF($P$1=2020,SUMIFS(MOOE!$BN$4:$BN$1000,MOOE!$A$4:$A$1000,">12/31/2019",MOOE!$A$4:$A$1000,"<02/01/2020",MOOE!$I$4:$I$1000,"MOOE20")))

from BM to BN and so on.

please techies.