Over the last few years, the simplicity of defining a global AUTOMATIC_SINGLE_SURFACE contact to treat the interactions between multiple parts of varying stiffnesses and element types has changed the way we model contact interfaces. They are not only simple to define but also promote better modeling since they hugely eliminate the need to manually identify contact surfaces which may be prone to errors. Legacy models which could have several hundred contact definitions (one-way and two-way types) can be extremely difficult to convert to a single contact definition since it requires a thorough review of manually defined segments. Here are some approaches that could be followed to migrate the legacy models to use a single consolidated contacts.

Approach 1
The simplest approach is to identify all the segments and use the parent parts to define a single surface definition. The main disadvantage is that segments which may not have been included in original contact definitions will now be included which may introduce new errrors especially if the newer segements are inter-penetrating.

Approach 2
The second approach, which is the recommended approach, is to take advantage of the output from D3HSP file. For every contact defintion (one-way or two-way) in the legacy models, LS-DYNA outputs the list of slave-segements (two-way) and a list of master-segments into the D3HSP file. We can consolidate all the slave and the master segments from the D3HSP file into one global segment list and use this segment list in a single surface definition. This avoids the addition of new segments. If we need the forces of interface as in the original legacy model, we can simple change the old contact defintions to a FORCE_TRANSDUCER contact which will provide a way to extract crucial interaction forces.
In some situations, the frictional constants can be different between various older contact interfaces. To model this, we can use the DEFINE_PART_FRICTION along with FS<0 to model part-pair based frictional coefficients.