Yes BlackSmith of Mammon can exchange All D, C, and B grade weapons for any other of the same grade and level (eg, low C Blunt exchange for low C Bow)
can even exchange High B grade now, so Lance for Damasac's etc
only works on full items not common and items cannot have an SA applied on them
Also you do not loose your enchanted status when exchanging so +5 kris could become +5 Dark Elven Bow
However if you upgrade your weapon you will loose your enchanted options. So +5 dark elven bow would become +0 Bow of Peril
hope that helps
all this info can be found on lineage 2 main website... somewhere lol