答案有点罗嗦，但能达到效果！不知道能不能加分阿！~

data a;
input id     v_n   a1 $;
n=_n_;
cards;
1001  1      .
1001  2      .          
1001  3      .          
1001  4     Y   
1002  1      .
1002  2      .  
1002  3      Y
;
run;
proc sort data=a;
by descending n ;
run;

data b;
set a;
laga1=lag(a1);
if a1="Y" then laga1="Y";
run;

proc sort data=b;
by n;
run;

data result;
set b;
drop n a1;
rename laga1=a1;
run;