Mistform Sliver

Oracle Text

All Slivers have "1: This permanent becomes the creature type of your choice in addition to its other types until end of turn."

Card Rulings

10/4/2004 Unlike most type changing effects, this one adds a creature type instead of replacing existing creature types.
7/1/2013 Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit.
7/1/2013 If the creature type of a Sliver changes so it’s no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.

Additional Information