Will becomes a genius and Smith becomes an old man.
Firstly, let me point out that this episode is liked by some fans...but not by me.
This episode and The Oasis are my two least favourite episodes of season one. Both hours deal with Smith transforming into something terrible or hard to swallow (a giant or an old man) so obviously I am not too comfortable with that theme in some of LIS.
Then again, Smith would transform into something else in season two's Cave Of The Wizards and season three's Collision Of The Planets and I was not bothered by that??
Whatever the case, Change Of Space does not do it for me, but some cool alien hardware and a Voyage to the Bottom of the Sea-creature make it watchable.