Hi Alison,
If I see the code, the system will mark a material as NEW if you have extended the material to a new plant i.e. an entry created in table BDCP2 with MARC, KEY and I. Now this entry will be created in table BDCP2 if you have included MARC, KEY in BD52 settings for /SAPSLL/MATMAS_SLL.
Now since you have mentioned that changed materials are appearing as NEW in GTS, it is most likely that you have MARC, KEY maintained in BD52 settings.
Please do post your comments if it helps.
regards,
Kul Vaibhav