Some improvements in the ACCESS receivers I've read about include lower latency, more channels (24 vs 16, only applies if using SBUS since no receivers have 16 ppm channels that I'm aware of), longer range (advertised as 2+ km vs 1.5+ km), better telemetry range, adjustable telemetry power, and OTA receiver firmware updates (the radio can flash new firmware onto the receivers without a physical cord).
It appears to be the future of FrSKY, so they will likely be supported long after ACCST support stops. But there's no need to replace existing receivers, unless there's a specific feature on an ACCESS receiver you're interested in that's not available on a receiver you already have.