Hello,
Changing that check box for Ext. number range for the material master is nothing but changing the configuration for the material master number range. And if that is far beginning (i.e. beginning of the project) means no materials should exist with external number range and including there is no stock should exist in that material. If that is the case you can change go to t-code: MMNR (directly into system where you want this change, because this is open client activity and it is manual transport) is case1.
case2. is you need to list out the materials or items with external number in the system and what is plan of creating them as material with internal number range? because we might require to delete external and creating them as internal number range.
Hope it clarifies..
Thanks,
Srinivas