Removing Exchange 2007 self signed certificate

I am having difficulties remove my old certificate even though have add a new one any help from you.

I am having several error messages on my exchange server. The certificate have already been generated and add via CSR through comodo. I have configure it add i can see it. The old one will expires in 3 days time.

ERROR MESSAGES WHEN REMOVING CERTIFCATE.
C:\Documents and Settings\Administrator\Desktop>Remove-ExchangeCertificate
-thumbprint "8002CFBF8D86EF471BF082CFF01A63100000DED"
Remove-ExchangeCertificate : The internal transport certificate cannot be remov
ed because that would cause the Microsoft Exchange Transport service to stop. T
o replace the internal transport certificate, create a new certificate. The new
 certificate will automatically become the internal transport certificate. You
can then remove the existing certificate.
Parameter name: Thumbprint
At line:1 char:27
+ Remove-ExchangeCertificate  <<<< -thumbprint "8002CFBBBB00000082CFF01A63100000AA"

EVENT VIEWER

An internal transport certificate will expire soon. Thumbprint: 8002CFBBBB00000082CFF01A63100000AA, hours remaining: 102
The STARTTLS certificate will expire soon: subject: mail.gorkana.com, hours remaining: 8002CFBBBB00000082CFF01A63100000AA. Run the New-ExchangeCertificate cmdlet to create a new certificate.