Tables change as products change. ALTER TABLE edits a table in place, keeping its rows:
ALTER TABLE products ADD COLUMN discontinued BOOLEAN DEFAULT false;
ALTER TABLE products RENAME COLUMN units_in_stock TO stock;
ALTER TABLE products DROP COLUMN cost;
ALTER TABLE signups_raw ALTER COLUMN age_text TYPE INTEGER USING TRY_CAST(age_text AS INTEGER);
A new column is NULL in existing rows unless you give a DEFAULT, which fills them in. Changing a type converts every existing value; USING says how, and without it one value that won't convert stops the whole change.
Syntax varies more here than anywhere else. MySQL writes MODIFY COLUMN to change a type, SQL Server uses ALTER COLUMN and renames columns with sp_rename. The portable way to change a type is four steps: add a new column, fill it with UPDATE, drop the old one, rename the new one.