  We study virtual isotopy sequences with classical initial and final diagrams,
asking when such a sequence can be changed into a classical isotopy sequence by
replacing virtual crossings with classical crossings. An example of a sequence
for which no such virtual crossing realization exists is given. A conjecture on
conditions for realizability of virtual isotopy sequences is proposed, and a
sufficient condition for realizability is found. The conjecture is reformulated
in terms of 2-knots and knots in thickened surfaces.
