ATLAS Minimum Bias Trigger Scintillator upgrade for LHC RunII

The Minimum Bias Trigger Scintillators (MBTS) delivered the primary triggers for selecting events from real LHC collisions with the smallest bias for the low luminosity LHC RunI fills from 2009-2013 (proton-proton, lead-lead and lead-proton collisions). MBTS also will provide key ingredients for the first RunII physics measurements at the new LHC proton-proton collisions energies (charge multiplicity, proton-proton cross section, rapidity gap measurements, ...). After more than 25fb-1 of proton-proton collisions delivered during RunI of LHC, MBTS detectors have been substantially upgraded for the RunII of LHC (starting in 2015). The upgrade strategy will be presented showing the scintillator replacement, the modified read out scheme, the optical measurements on RunI scintillators assessing the degradation due to the dose received and how the new simulation is being implemented to take into accounts all the modifications foreseen for RunII.