I've two tables in mysql, both of them are type InnoDB and there's a current foreign key from the first one to the 2nd.

However, the foreign key was produced with no cascading down behavior. The table is big, and then any changes to indexes or new secrets locks the table for ~twenty minutes.

Can you really add "ON Remove CASCADE" behavior for an existing foreign key without shedding the important thing and re-creating it?

Regrettably, no. On Remove as well as on Update are systemic characteristics from the foreign key itself and may simply be specified when designing the constraint.