Hello Vidhyadhar,
there is really no way to change the PMP afterwards except from reversing the postings - and this makes sense as the posted valuations would actually become wrong according the new PMP.
The workaround via a transfer posting with tpm15m is available with FIN_TRM_LR_FI_AN, which is EhP3 (not EhP5), please check the relevant release notes.
So if an upgrade to 603 is not feasible, the reversal is the only option.
Best regards,
Marie