Ultimately, fiber types are defined by the myosin isozyme they express. Endurance training causes your type IIx fibers to begin to express type IIa myosin, and your type IIa fibers to express type I myosin. At some point, you’d say that the fiber had been “converted”, but hybrid fibers co-expressing two, or even all three, of the major isozymes can exist at any time.
Converting IIx fibers to IIa fibers is easy; converting IIa fibers to I fibers is difficult (to the point that controversy still exists in some circles).
Going the other way, sprint training may (or may not) increase type IIa myosin at the expense of type I myosin, but like endurance training it causes rapid conversion of IIx fibers into IIa. The only way to have lots and lots of IIx fibers is to become paralyzed (not recommended).
Last comment: there are multiple determinants of the contractile properties of muscle beyond just the type of myosin that is expressed. The latter is, however, what science ultimately hangs its hat on when defining a fiber’s “type”.