Network layer.
LAN technology is on data link layer. Ideally a layer should only care about a layer adjacent to it, and transport layer shouldn't be affected by change in data link layer, because network layer is present between them.
Edit: (I am adding an edit here because I don't have permission to comment, I hope its okay)
its also on PL as well as on DL layer
That's true.
LAN technology is on Physical and data link layer. These two got changed completely on changing the lan technology. Now the data link layer is sending information in some format to the network layer above it. I am assuming this format also got changed when lan technology got changed. Now we need to make some changes in network layer so it knows how to communicate with the new data link layer. (If the new data link layer and old network layer already knows how to communicate with each other, then even network layer need not change)
everyone adjacent to each other
Transport layer is not adjacent to data link layer. Transport layer (ideally) only needs to communicate with layer adjacent to it which are network and the application. Our network layer wasn't completely replaced, so transport layer knows how to communicate with the current network layer and the transport layer doesn't need to change.
Ans: Physical and data link layer got changed (obviously), network layer most probably needs changing (otherwise I am misunderstanding the question). Transport layer doesn't needs to change.
