Whenever the number of your monsters removed from play changes, re-calculate the DEF of “Lost Guardian”. (Just like “Shadow Ghoul”, etc.)
When a monster is removed from play, it goes to a “removed zone” (for lack of a better term) of the original owner. If the opponent controlled it, it will return to the opponent’s side of the field when it returns to player, however it is considered to be in the original owner’s “removed zone” while out of play. This is important for effects such as “Lost Guardian”.
Example: Player A controls 2 monsters. Player B activates “Change of Heart” to take control of one of them, then activates “Different Dimension Gate” to remove both from play. Both monsters are in Player A’s “removed zone” while removed from play (so Player A’s “Lost Guardian” would have 1400 DEF). When “Different Dimension Gate” is destroyed, the monster Player B took control of will return to Player B’s side of the field (if possible, if not it is destroyed) before returning to Player A’s side of the field.
Monsters removed from play and kept face-down, like with “Different Dimension Capsule” or “Lightforce Sword”, do NOT count towards the DEF of “Lost Guardian”.