A change of a field's type is always possible when there are no existing values tied to the definition (no values entered for a product, a page etc).
However, when existing values are already in place, care must be taken before carrying out a change. Please see the below table for how field types can be changed.
Also note that while something is possible from a technical standpoint, it doesn't mean it's a good idea. But ultimately, this is for you to decide.