Converting CHAR to BIT
Can you be more specific on "quite large"?  In general you're right, you shouldn't just 'do it live' in production with any change.  Do you have any kind of QA or test environment you can do a run on first?

This isn't the most sexy ninja one line approach but you could


Add a new bit column
Issue updates setting the new column = if( length(old_column)=16,1,0)
Alter table to drop old column and rename the new column