I am not an expert in this area and I welcome others to correct me, but it looks like the replacement of IMPs with TCP/IP requires a little more background.
IMPs, or Interface Message Processors, were packet switching hardware with their own special software. IMPs handled the Physical Layer, the Data Link Layer, and the Network Layer of the Protocol Stack. So in other words, IMPs were hardware combined with hardware that provided primitive network communications from node to node.
Then ARPANET introduced the Network Control Program (not to be confused with the Network Control Protocol), which handled the Transport Layer of the Protocol Stack. On top of NCP the Application Layer of the stack ran, (e.g. file transfer and email). Here is a more technical description of the necessity for the NCP:
The 1822 protocol proved inadequate for handling multiple connections among different applications residing in a host computer. This problem was addressed with the Network Control Program (NCP), which provided a standard method to establish reliable, flow-controlled, bidirectional communications links among different processes in different host computers. The NCP interface allowed application software to connect across the ARPANET by implementing higher-level communication protocols, an early example of the protocol layering concept incorporated to the OSI model.
Source : https://en.wikipedia.org/wiki/ARPANET#Software_and_protocols
So at this point we had the following Protocol Suite:
Application layer [On top of NCP (e.g. email and file transfer)]
Transport layer [NCP (e.g. AHHP and ICP)]
Network layer [IMP]
Data link layer [IMP]
Physical layer [IMP]
Then, NCP was replaced for TCP/IP:
On January 1, 1983, known as flag day, NCP was officially rendered obsolete when the ARPANET changed its core networking protocols from NCP to the more flexible and powerful TCP/IP protocol suite, marking the start of the modern Internet.[2][3][4][5] However, NCP provided a slightly different service than TCP/IP by forming a network of networks. Hence, the transition meant a loss of functionality and increased complexity in TCP/IP.
Source : https://en.wikipedia.org/wiki/Network_Control_Program#Transition_to_TCP.2FIP
Interestingly, IMPs were still in use for the next six years, until DARPA got rid of ARPANET:
IMPs were at the heart of the ARPANET until DARPA decommissioned ARPANET in 1989. Most IMPs were either taken apart, junked or transferred to MILNET. Some became artifacts in museums; Kleinrock placed IMP Number One on public view at UCLA.[6] The last IMP on the ARPANET was the one at the University of Maryland.
Source : https://en.wikipedia.org/wiki/Interface_Message_Processor#History