Retaining encryption during Exchange upg

We recently migrated from Exchange 5.5 SP4 to Exchange 2000 SP2. We were running Key Management Server issuing X.509 V3 certificates. I installed an Enterprise Root CA and set up KMS in Exchange 2000. I wanted to know if it is possible to use the old encryption keys in Exchange 2000 so
that users can view their old encrypted e-mail. I tried importing the old digital IDs and it failed and I also tried enrolling the users to try and
read the encrypted e-mails but that also didn’t work. Is therea workaround?