How to start abrupt-4xCO2 from an existing B1850cmip6 run (CESM2.1.5)

[CCl]

Hi everyone！
I’m using CESM2.1.5 and have an existing coupled B1850cmip6 case (piControl-style) that has already produced restart files. Now I would like to run an abrupt 4xCO2 experiment starting from one of those restart dates.

From the CESM2.1.5 compset documentation, it looks like BCO2x4cmip6 is specifically designed for the CMIP6 DECK abrupt4xCO2 experiment (abrupt quadrupling of CO2 with other forcings kept at 1850 levels).

My question is about the recommended workflow:

1. Create a new case with compset BCO2x4cmip6 (same resolution as my B1850cmip6 case) and start it as a branch run from my B1850cmip6 restart files (set RUN_TYPE=branch, RUN_REFCASE, RUN_REFDATE, RUN_REFDIR, etc.).

OR

1. Keep the same compset (B1850cmip6) but set co2vmr = 1138.8e-6 in user_nl_cam, run ./preview_namelists, and proceed. I saw forum guidance that changing co2vmr and re-running preview_namelists should propagate into CaseDocs/atm_in.

Do you recommend option (1) or (2) for an abrupt-4xCO2 experiment？

Thanks very much for your time and advice.