perl -pi -e 'if(/<level1(\sid=\".*\")>/){$a=$1};if(/<level2>/){$_=~s/(
+<level2)>/$1$a>/g}' input

or

perl -pi -e '$a=m/(<level([12]))(\sid=\".*\")*>/;if($a){$2==1?$b=$3:$_
+=~s/($1)/$1$b/g;}' input

or

perl -pi -e '$a=m/(<level(1|2))(\sid=\".*\")*>/;$a?$2==1?$b=$3:$_=~s/(
+$1)/$1$b/g:next;' input

or

perl -pi -e '$a=m;(<level(1|2))(\sid=\".*\")*>;;$a?$2==1?$b=$3:$_=~s;(
+$1);$1$b;g:1;' input
[download]

or golf by monsieur_champs

perl -pi -e 'm;(<level(1|2))(\sid=\".*\")*>;;$&?$2==1?$b=$3:$_=~s;($1)
+;$1$b;g:1;' input
[download]

sorry =) -1 char

perl -pi -e 'm;(<level(1|2))(\sid=\".*\")*>;;$&?$2==1?$b=$3:$_=~s;($1)
+;$1$b;g:1' input
[download]

insane !!! 33 chars

perl -pi -e '/l1(\sid=.*)>/?$a=$1:s;l2>;l2$a>;' input
[download]

=)

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