Hi Jean-Christophe! Thank you for your feedback. You should be able to remove an asset from a group and keep the triggers/animations as Stephan mentioned. Since it's hard to select objects from within a group on the slide, I use my timeline to grab the exact items I want to remove. The remaining group stays intact and so do the triggers and animations.
You're right that you can't just add assets into an existing group and keep the animations and triggers. That would be nice! We don't have plans for this feature yet, but you're welcome and encouraged to share your thoughts by submitting a feature request. If anything changes, I’ll share it here.
https://articulate.com/support/contact/feature-request