Alter table with casting
You can do it using a single statement.




CREATE TABLE T (val decimal(18,3));






INSERT INTO T VALUES (1.25);
INSERT INTO T VALUES (12.526);
INSERT INTO T VALUES (1000.25);






SELECT val FROM T;



|      val |
| -------: |
|    1.250 |
|   12.526 |
| 1000.250 |






ALTER TABLE T MODIFY COLUMN val bigint;






SELECT * FROM T;



|  val |
| ---: |
|    1 |
|   13 |
| 1000 |



dbfiddle here