The HADES experiment at GSI will soon provide data on the production and properties of ∆ baryons from Au+Au reactions at 1.23 AGeV. Using the UrQMD model, we predict the yield and spectra of ∆ resonances. In addition we show that one expects to observe a mass shift of the ∆ resonance on the order of 40 MeV in the reconstructable ∆ mass distribution. This mass shift can be understood in terms of late stage ∆ formation with limited kinetic energy. We show how the mass shift can be used to constrain the kinetic decoupling temperature of the system.