Step 1:
Generated and simulate the required events in CMSSW_10_6_20 (Filters applied here) cmsRun HToAA4Tau_M_*_GEN_SIM_cfg.py
Step 2: digitization and premixing events in CMSSW_10_6_20 (pileup applied here) cmsRun HToAA4Tau_DIGIPremix_cfg.py
Step 3:
Add HLT information in CMSSW_10_2_16_UL using generated events in step 2 because HLT menu is not available in 10_6_20 cmsRun HLT_H_AA_4Tau_RunIISummer20UL18HLT-00003_1_cfg.py
Step 4:
Convert raw to AODSIM back into CMSSW_10_6_20 using generated events in step 3 cmsRun raw_to_AODSIM_cfg.py