you just have to open the box/cover, remove the EZ mate and put in the KD. In this case a #148 coupler is probably the best match.
The #147 mentioned above is compensated for lower mounting and I assume it is suggested to compensate for the thickness of adding a coupler box, However, I don't think in this situation a new coupler box is necessary.
If it's an older version 90's version like this...
Here's a tip to guard against losing those
tiny springs that operate the Kadee knuckles.
Put a TINY bit of glue on the spring base.
That will keep the spring from embarking on
a trip to the moon if you should have
a very hard coupling action. It also makes
it easier to reset the spring.