I can't see a direct replacement for the T400.

In most cases you may use a S7-1500, S7-1500T, Drive Controller or Open Controller. (positioning or synchron operations).

Some other applications, e.g. winder, you may solve with the internal free function blocks of the SINAMICS S120 (I think the DCM has free blocks too). You may search for a Application Example or do it yourself. 

In any case you have to create a new program. There is no migration.

The S7-1500T offers several synchronization modes. With MC_GearInPos you can start a synchron movement. This should cover all modes of the T400 position sync modes.

The S7-1500 (without T) offers only the MC_GearIn. This is a synchron operation too, but without definition of the angle between the drives. 

If an encoder is connected to the DCM it should be possible to transfer it's data via PROFIBUS to the S7-1500. Because I don't know the DCM in detail, I can't give a clearer answer.

A separate encoder for the S7-1500 is a technical better solution, due the DCM does not support isochrone operation (It is always synch with the power line.) 

You can use a PROFINET encoder (In best case on a 2nd PROFINET bus, which uses IRT (Isochrone bus) or a TTL/HTL / SSI encoder with an interface module (e,g, FM Pos Input, FM Pules, etc)