I'm not sure even if the latest generation of the D11R was Tier 2. I get the impression they were Tier 1 but I'll need to dig a bit more to confirm that.
The reason I say that is the last generation of 11Rs (before the T model came along) that were built with an electronic 3508 engine from the get-go have a C32 retrofit available, but that retrofit is only a Tier 2. Now that would not make much sense to me if the original engine was also a Tier 2 because the usual reason for a retrofit is to install an engine meeting a higher EPA emissions level, not one the same as the original engine.
EDIT: Well that didn't take very long. Even the current model D11T tractor is only rated EPA Tier 2, probably because of the 850BHP power output. I confirmed this from the EPA standards. Engines rated above 750 BHP (which include the OP's D11N but interestingly enough not his 650BHP D10N machines) were required to meet EPA Tier 1 from 2000-onwards and EPA Tier 2 from 2006-onwards. Since 2006 nothing has changed regarding the emissions requirements they have to meet.
https://dieselnet.com/standards/us/nonroad.php
All electronic D11R tractors were rated EPA Tier 1, and the C32 retrofit is designed to take them to the same Tier 2 emissions level as the latest D11T. I have searched again and can find nothing regarding a C32 retrofit for a D11N.
Also, with your 11N having been originally built in 1989 it pre-dates emissions regulations by about a decade and I guess could be regarded as being Tier "0" emissions, hence the upgrade retrofit to a Tier 1 electronic 3508 engine that was offered back in the day.