The coupler is not the issue. The circuit board in the LC+ engine receives the command signal from the remote and sends the signal to the coupler. The remote DOES NOT talk to the coupler.
Same principle with TMCC engines. The boards do the talking and send voltages to the components to operate.
So, if I read you post correctly, just adding a LC+ coupler to the K-line engine won't make the coupler work with the remote.
There are no conversion kits for non LC+ engines to make them LC+. G