Changing INT to BigInt

If one or more of those columns have no constraints against them (like a foreign key, index, default, rule, etc), you should be able to change each one quickly by doing

ALTER TABLE monster ALTER COLUMN MyIntCol1 bigint

Management Studio's change SQL is rarely the most efficient and tends to favour temporary tables for anything modifying an existing column.

Not sure if this will help, but try this:

1 - create a new bigint column in the table
2 - update that new column with the values from the int column
3 - delete the int column
4 - rename the bigint column