Shift Underlining a row in an array

For this code I just want the underline of the elements to shift to the second row when I start calculating the second row of the resulting matrix. Is there a way to do that? (And I also want the underline to disappear from the first row.)

documentclass[aspectratio=169,xcolor=table]{beamer}
 definecolor{lava}{rgb}{0.81, 0.06, 0.13}
definecolor{myblue}{rgb}{0.0, 0.30, 0.60}
newcommandx[1]{color{myblue}{mathbf{#1}}}
newcommandy[1]{color{lava}{mathbf{#1}}}
begin{document}


begin{frame}{Matrix Inverses}

underline{Understanding Elementary Matrices}
begin{itemize}
item if $mathbf{A}=   left[ begin{array}{rrrr}
 a & b & c 
d    &e &f  
g & h & i 
end{array}right],$ then $ldots$
item[]

$
mathbf{E}_1mathbf{A}=setlengtharraycolsep{4pt}
   left[
begin{array}{rrrr}
 underline{ 1}rlap{underline{~~~~}}  & underline{0} rlap{underline{~~~~}} & underline{ 0} 
0     & 1 & 0   
  x{ -4}  & x{ 0}  &x 1  
end{array}
right]
left[ begin{array}{rrrr}
 a & b & c 
d    &e &f  
g & h & i 
end{array}right]= onslide<2->{hspace{-3em}
left[ begin{array}{rrrr}
pause 1times a pause +0 times d pause +0 times g =y a 
& pause 1times b pause +0 times e pause +1times h pause =y b 
& pause 1times c pause +0 times f pause +0 times i pause =y c pause  
pause 0 times a pause +1 times d pause +0 times g =y d 
& pause 0 times b pause +1 times e pause +0times h pause =y e
& pause 0 times c pause +1 times f pause +0times i pause =y h pause  
pause x{-4} times a pause + x{0} times d pause + x{1}times g =y d 
& pause x{-4} times b pause + x{0}  times e pause +x{1} times h pause =y e
& pause x{-4}times c pause + x{0}  times f pause +x{1} times i pause =y h pause  


end{array}right]}
$
end{itemize}
end{frame}end{document}