$ echo 'aa@aa' | sed -E '/^(a+)/!b;s/^(a+)(.*)\n/\1\n\2/;h;s/^(a+).*$/\1/;s/./A/g;G;s/\na+//'
aa@AA

$ echo 'aabbb' | sed -E '/^(a+)/!b;s/^(a+)(.*)\n/\1\n\2/;h;s/^(a+).*$/\1/;s/./A/g;G;s/\na+//'
AAbbb