The relevant parts of my config:
import:
move: yes
plugins: convert
convert:
auto: yes
max_bitrate: 192
format: mp3
If the imported mp3 file has a bitrate under 192 it gets moved unconverted as desired.
If it's higher than that the old file remains inside the source folder after the successful conversion and the new file get's copied to the destination folder. It acts like the copy flag. The output shows:
convert: Encoding D:\Test\Test.mp3
convert: Finished encoding D:\Test\Test.mp3