SurgeMail will detect an existing DMail configuration and create a SurgeMail configuration based on the dmail.conf file. In general this will successfully upgrade an existing DMail installation.
However, SurgeMail is not 100% compatible with DMail and live upgrades should be attempted with caution. (e.g. first backup your existing system so that you can back out of an upgrade)
The suggested upgrade procedure is to copy your existing dmail.conf file to a test system and install SurgeMail on the test system to ensure that SurgeMail will upgrade your DMail configuration. The SurgeMail installation scripts will warn you about any ini settings that it thinks it cannot convert and might be serious. If you use include files in your dmail.conf or have scripts that modify dmail.conf you should definitely hesitate as SurgeMail uses a completely different ini file format.
Also note that when DMail is upgraded external modules such as authentication modules and their databases will remain in their original location. It is important to check your dmail.conf and surgemail.ini files before deletion of any files relating to your DMail configuration.
If you find features that are missing that you really need let us know and we'll try and get them added.
The license upgrade from DMail is not free but we will give a 50% discount to any existing DMail users wishing to take advantage of the new features in SurgeMail.