Urborg

• WWizards of the Coast2004-10-04
  Official Ruling
  The ability can be used on a creature without First Strike or Swampwalk but has no effect.
• WWizards of the Coast2008-04-01
  Official Ruling
  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.
• WWizards of the Coast2009-10-01
  Official Ruling
  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.
• WWizards of the Coast2009-10-01
  Official Ruling
  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.
• WWizards of the Coast2009-10-01
  Official Ruling
  Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.

Rulings via Scryfall