Shell Programming and Scripting

awk -F'//' '
/:16S::GENL/{
  flag=""
}
/:16R::GENL/{
  flag=1
}
flag && /:20C::RELA/{
  val1=$1;
  val2=$2;
  getline;
  val3=$0;
  getline;
  if($0 ~ /:20C::MITI/){
    sub(/.*/,$2,val2)
};
  print val1 FS val2 ORS val3 ORS $0;
next
};
1
'    Input_file

awk '
/:20C::RELA/    {T = $0
                 sub ("^" $1, _, T)
                }
/:20C::MITI/    {sub ("/.*", T)
                }
!NF             {T = ""
                }
1
' FS="/" file

awk '
        {for(i=1; i<=NF; i++)   {if ($i~/:20C::RELA/) R = i
                                 if ($i~/:20C::MITI/) M = i
                                }
         if (M)                 {$M = $R
                                 sub ("RELA", "MITI", $M)
                                }
         R = M = ""
        }
 1
' RS= FS='\n' OFS='\n' ORS='\n\n' file