Is this possible? Assuming I know the exact name of the spell class as it was added by an earlier class feature.
You can only set the base proficiency when you add the spell class originally.
Related: Is it possible to change the spells per day of an existing spell class?
-- Spell slot functionality - "add" action currently supported
-- Format: SPELLSLOT <action> <spell class name>|<spell slots to add>
-- <action> - add currently supported.
-- <spell class name> - The name of the spell class to match on the actions tab.
-- <spell slots to add> - how many spell slots to add - in the form <rank>:<number>. For example: C:1 to add 1 cantrip, 1:1 to add one rank 1 slot. Currently only supports one slot rank - use SPELLSLOT add automation multiple times if needed.