[exim] Update OpenSSL

In my Ubuntu Server 11.04 with Exim 4.74, OpenSSL 0.9.8o
I'd like to update OpenSSL to support TLS1.1 or TLS1.2 in the encrypted
SMTP connections.
Can i keep the present private key and certificate or i need to
regenerate them after the update?
What are the correct steps to update OpenSSL ?
Can i rollback to OpenSSL 0.9.8o if something goes wrong ?

Regards