Before that one takes off on folks, unless you expose magnetic media (especially a HDD platter) to a strong magnetic field it isn't going to spontaneously demagnetize.
Every sector of every track actually has bits on it to guide the head on a read/write approach to get in on track. These are not accessible to the head for writing operations (consumers anyway - they are permanently written to the platter as part of the manufacturing process); so if these just demagnetized (which they don't), "refreshing" would be a moot point anyway because the heads would have no clue where they are.
For a 3.5" floppy, the areal density is so low (with such a massive margin for error) that the magnetic concerns pale in comparison to exposure to heat, dust, and general bad environment. Those I would recommend copying to something more impervious to the elements (they are essentially a more rigid version of magnetic tape and subject to the same concerns).