suppose u want to change column a in table t, from varchar(20) to numeric(20) and change its name from a to b,
u can use

ALTER TABLE t CHANGE a b numeric(20);

value in the column will be truncated, if the memory of new type is not big enough to store this value;


As for primary key, if u want to change its type from one to another, u should make sure that there won't be two duplicate value in that column after u do the transformation. otherwise, it will refuse to do the job