Q: Is it possible to play a unit from your hand even if all of your zones are full?
A: Yes, it is possible.
Explanation: When doing a normal summon, you are allowed to summon the unit in one of your zones already occupied by another unit, unless there is some restriction involved that specifies otherwise.
If you do normal summon a unit into a zone already occupied by a unit, then once the new unit enters play, then the pre-existing unit is put into the graveyard during the next time rules effects are resolved.
A7 "Duchess" the Brawler (2021GB03-046)
Q: If the unit chosen for the "Roaring Thunder Wave" ability deals combat damage to an enemy unit, does it lose the increase to its STK?
A: No, dealing combat damage to an enemy unit does not remove the increase to STK from this ability.
Akagi of the Centipede (2021GB03-066)
Q: If an opponent plays a
A: Yes, since Discount doesn’t actually reduce the level of the card,
Aras, Guardian of the Divine Winds (2021GB03-004)
Q: When using the "Return from Valhalla" ability, can you special summon a face down unit in your damage zone?
A: No. A face down card in your damage zone loses all information printed on its other side, hence no longer fulfills the conditions required by this ability.
Ascendant's Wrath (2021GB03-053)