I'm looking for a verb that means implementing the same functionality that already exists in another system. "To move" seems to imply that it's no longer present in another system. Is "to transfer" the best choice? (e.g. I'm working on XXX from system A to system B)
I think you may be looking for to port (from Oxford Dictionaries):
[WITH OBJECT AND ADVERBIAL OF DIRECTION]
Transfer (software) from one system or machine to another:
"the software can be ported to practically any platform"
However, this dictionary definition doesn't give a full picture. Porting software more commonly refers to taking software that is compatible with one system, and modifying it so it becomes compatible with another. You might port a Windows program to Mac, or a Playstation game to Wii.
You can also port features from one piece of software to another, or from one version of a piece of software to another version of the same piece of software. This is usually done to introduce new features to older versions. In that case, it is known as backporting. For example:
Backports provide drivers released on newer kernels backported for usage on older kernels. Always use the latest stable release! The project started since 2007 and was originally known as compat-wireless, evolved to compat-drivers and was recently renamed simply to backports.
In this example, device drivers introduced in newer versions of the Linux kernel have been adapted (backported) so that they work with older kernel versions.
To port is generally used when there are modifications being made to the software, rather than just swapping out the software for something else.
In your specific example, there is another term that can be used:
I'm working on migrating from system A to system B.
With its definition (from Oxford Dictionaries):
(With reference to computer users) change or cause to change from one system to another:
[NO OBJECT]: "Customers are migrating from mainframes to client-server environments."
[WITH OBJECT]: "Save time by efficiently migrating data to secondary storage systems"
Transfer (programs or hardware) from one system to another:
"The system will allow users to migrate applications across environments."
To migrate may be used in certain places like to port, but unlike to port it isn't necessarily implied that the software or hardware is being modified. Rather, it implies that the software or hardware is being swapped for something else entirely.
The normal term used for this is porting. This is definition 5 from the Oxford Dictionary:
computing: Transfer (software) from one system or machine to another:
Your sentence should therefore be:
I'm working on porting from system A to system B