In SQL Server Management Studio, when changing an existing field in design view from
Just use T-SQL script instead of the visual designer to achieve your goal:
ALTER TABLE dbo.YourTableNameHere ALTER COLUMN YourColumnNameHere DECIMAL(18, 14)
and you should be fine.
The visual designer takes the extra careful route of creating the table with the new structure and then copying over all the data - it works, but it's tedious. Normally, as long as you don't truncate a column (make it shorter), you can definitely change the column's datatype "in place" using a T-SQL statement.
Also: by default the SSMS designer is extra careful and won't allow any changes that require a drop-and-recreate table cycle. You can disable this extra carefullness in
Tools > Options and then in this dialog box:
If you uncheck that option, you will be able to do "destructive" changes in your visual designer