2 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

When this card's effect places 1 of my other blue Digimon as this Digimon's bottom digivolution card, what happens to that Digimon's digivolution cards?
They're trashed.
Ruled Mar 28, 2024
When this card's effect places 1 of my other blue Digimon as this Digimon's bottom digivolution card, if that Digimon has [BT9-109]/[X Antibody] in its digivolution cards, what happens to that card?
It's not being trashed by the effect, it's being trashed because of the rules, so [BT9-109]/[X Antibody] does still get trashed.
Ruled Mar 28, 2024
Got a line that uses Mermaimon? Map it in the builder and share it with a link.
Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.