You cant have two routable maps covering the same region,nuvi wont see one of them,deselect one or ,better ,delete one.
Deleting the preloaded map wont give any issue , backup it on pc the old one along .unl and .gma files and delete, unlock and rename the new one like the previous one and copy it to the same place. But i donnow which map are you talking about and which additional files you need to make it working fine.
Take a look at this thread
[Only registered and activated users can see links. ]
Every CN eg. has its own additional files to copy in the right path, you can take a look at device.xml in Garmin folder to figure it out.