I dare say it would be a proposition for the topologists. The best way to
conceptualise an open universe in three dimensional space is by imagining
a "saddle". Changing from a closed universe to an open would be a similar
problem to morphing (for want of a better term) a sphere into a saddle.
Changing from an open to a closed universe is, of course, a nice thought
experiment but the most practically unlikely an engineering feat as has ever
been dreamed up!
-- transient@mindless.com