6556 is the best of HSDIO and the only next level replacement is the PXIe-6570/6571 but they are completely different architecture and requires complete redevelopment and many features of 6556 are not present in 6570/6571
The next lower replacement is the 6555 HSDIO.
Is that card going obsolete? if then literally no other option match the features of that card (depends on what exactly you use that card for)
-Santhosh Semiconductor Validation & Production Test Soliton Technologies NI CLD, CTD LabVIEW + TestStand + TestStand Semiconductor Module (2013 - 2020) NI STS for Mixed signal and RF