The ability can be used on a creature without First Strike or Swampwalk but has no effect.

If a creature loses first strike after first strike combat damage has been assigned, it won’t deal damage during the second combat damage step.

Causing a creature to lose swampwalk is useful only during the declare attackers step or earlier. Once the declare blockers step begins, it’s too late to block the creature.

Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.

You choose whether the targeted creature loses first strike or swampwalk as the ability resolves. If the creature has both abilities, it will lose just one of them.