The enchanted creature’s toughness doesn’t turn into loyalty counters. Its toughness is its loyalty, and that value is modified appropriately as the enchanted creature is dealt damage.

Loyalty costs on enchanted creature treat putting or removing loyalty counters on/from it to changing its toughness instead. A [+1] cost on a loyalty ability, for example, effectively has a cost of “increase enchanted creature’s toughness by 1,” though it still counts as a loyalty ability. Similarly, a [−X] cost becomes “Reduce enchanted creature’s toughness by X.” You can’t pay a cost that would reduce enchanted creature’s toughness to less than 0.

Other effects can still modify enchanted creature’s power and/or toughness as they would any other creature, and their durations are not affected. For example, Invigorate can give the creature +4/+4 until end of turn. While its toughness is boosted, that toughness can be reduced as a cost to activate its loyalty abilities. However, once the turn ends and Invigorate wears off, if the creature’s toughness is 0 or less, it would be put into a graveyard as a state-based action.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.