find and replace variables for multiple xml tags.

Hello All,

    i have some tagged variables in xml which i want to modify from source and target using sed or awk.

source:

  <cdb_name oa_var="s_cdb_name">EBS2SIT</cdb_name>
  <cdb_unique_name oa_var="s_cdb_unique_name"/>
  <global_db_name oa_var="s_dbSid">PEBS2SIT</global_db_name>
  <db_name_lower oa_var="s_dbSidLower">pebs2sit</db_name_lower>

    should be like below in target: ( copied the same source file to target and modified)

  <cdb_name oa_var="s_cdb_name">EBSPAT2</cdb_name>
  <global_db_name oa_var="s_dbSid">PEBSPAT</global_db_name>
  <db_name_lower oa_var="s_dbSidLower">pebspat</db_name_lower>

    LIKE WISE HAVE MANY WITH IN TAG

    I CAPTURED USING AWK the above values in variable

SOURCE:
S_PDBNAME_C= PEBS2SIT,
S_PDBNAME_S= pebs2sit,
S_UNIQ=EBS2SIT_12345,
S_CDBINST= EBS2SIT2,
S_INSTNAME= EBS2SIT

TARGET:
T_PDBNAME_C= PEBSPAT,
T_PDBNAME_S= pebspat,
T_UNIQ= EBSPAT_12345,
T_CDBINST= EBSPAT2,
T_INSTNAME= EBSPAT2

    used below to modify the source and target.

sed -i 's/$S_PDBNAME_S/$T_PDBNAME_S/g' $S_FILE

    challenge is although command is successful but the value in target file is just the same
    i need to directly modify the tagged variable with out creating temp variable or create 
    temp variable and modify the file with source and target variable.

sed -i 's/$S_PDBNAME_S/$T_PDBNAME_S/g' $S_FILE

outputs :
<global_db_name oa_var="s_dbSid">PEBS2SIT</global_db_name>

please share some solution I'm really exhausted with all combinations avail on google.

thanks in advance.
any input from your end is really valuable for me...

regards
Rahman Syed