The main differences between named and classic modes are the way in which they are implemented (globally for named while both globally and in interface mode for classic) as well as the addition of a sixth K value for the metric.
On the back end, that is, concerning the way that the routers interpret and operate in EIGRP, very little has changed. So a named mode EIGRP router can communicate with a classic EIGRP router. Since you configure the AS number under the EIGRP name configuration, if the AS is the same, they will communicate.
Now the only restriction is the issue with the K values. In order for EIGRP named and classic to interoperate, the number of K values must be the same. There are two possibilities.
One is that the classic EIGRP is being run on an older version of IOS which supports 5 K values. Named EIGRP configured router that receives EIGRP packets with 5 K values will revert to functioning with 5 K values and will not advertise the sixth.
The other possibility is that classic EIGRP is configured on a newer IOS version will actually advertise a sixth K value, but will not have it participate in the metric calculation. This is why you may see a K6 value in a wireshark capture of an EIGRP exchange. This will allow classic EIGRP to successfully interface with a named EIGRP configured router, without actually using that 6th K value for the metric.
I hope this has been helpful!