Converting from Tier 1 to CET1 is extremely important when the FHFA capital rule is so heavily weighted to CET1 capital. New JPS cannot be issued if existing JPS are not converted to CET1. GSEs, their FAs and FHFA will want the flexibility to offer new JPS in the future.
This was explained many times in this board. JPS is not counted as capital. Commons are counted as capital. Converting the JPS to Commons will increase the capital by about $32B. With annual earning of about $20B, the conversion will speed up the capital building by about 1.6 years.