Great question! Here's what the wiki says about Bieniemy's return and what it means for the Chiefs' scheme.
Short answer: The core scheme won't overhaul — but the Chiefs are clearly looking for a refresh.
Bieniemy was KC's OC from 2018–2022, the peak of the Mahomes/Reid era (three Super Bowl trips, two wins), so he's deeply familiar with the system. He runs West Coast/Reid-style passing concepts with heavy pre-snap motion and condensed formations (from the Eric Bieniemy OC page). His familiarity with Mahomes and Kelce should make for a quick transition.
What's different this time? The Andy Reid page notes that after a 2025 season where Reid "struggled to find the right formula," the front office made a deliberate bet on Bieniemy — along with RB and WR coaching changes — which "signals need for tactical adjustments." At 68, there are questions about whether Reid can refresh the scheme with more under-center run game usage and clever design to create easy yardage.
So don't expect a new offense. Expect the same foundational system, but with Bieniemy's voice back in the room to inject new wrinkles — especially in the run game and pre-snap motion packages — to get the offense back to its elite form.