Is there any quick way to migrate from postfix to exim? If there is anything that can help me to retain the existing webmin configuration for user accounts etc and continue running exim would be a great help.
As long as you are configuring Posfix and Exim via a generalized frontend like Webmin, Plesk, Confixx and others there is no difference between the both mailservers. When you have special requirements that you can only handle with the powerful Exim configuration then Webmin is not helpful anyway.
Because of the different possibilities to reach the same configuration in each MTA there is no default (nor quick) migration path. This depends on too many factors. Where do you store your accounts in Postfix (MySQL, cdb, hash-tables, LDAP, ...)? And where do you want to migrate them to? Where is your authentication mechanism (Courier-authdaemon, dovecot, ...)? Are you going to migrate it, too?