The right place to build WHERE clause is in BeforCursorFill method. BUT DO NOT Change SelectCmd, only change cSelectCmd parameter value.
Better DO NOT CHANGE anything just use Parameters:

this.SelectCmd = [SELECT * FROM YourTable WHERE SomeField = ?lePar1 AND OtherField = ?lePar2]

*** Then:
lePar1 = 1000
lePar2 = [BlahBlah]
oCA.CursorFill()
BROWSE NORMAL
lePar1 = 2000
lePar2 = [BlahBlah 800]
oCA.CursorRefresh()