Thank you sir, Amuk Murugul.
you provide some very important clues. My little bit of research suggests the expansion of Islamic Demak at the expanse of the old Hindu states and kingdoms, including Cirebon and Banten, in the 16th century could be a trigger. The fall of Melaka in the same century, leading to trading ships landing at Pattani instead could also be a factor that led the Javanese keris forms to the North.
A pertinent question could be why did the keris form not take root elsewhere closer like Palembang, Minang areas or even the Southern Peninsula? My current thinking is that these areas could already have existing incumbent kris cultures which were very strong, and hence, more inclined to assimilate parts of the influences rather than adopt wholesale.
The events in the 16th century could have led to an exodus of people, including keris-makers out of Northern Java. Or it could simply be the new flow of trade after Melaka's fall. I hear of a Kampung Jawa even in Kelantan. This sets the stage for the subsequent evolution of the coteng and tajong keris from the northern Javanese forms, and I note that the earliest coteng/tajong seemed to date no further back than the 17th century.
I will check out the Sunda leads. I will be glad to hear your views.