MacBook Air messages
what if I logged out of my acc, and signed in on my new account in messages and see my old messages (on my laptop). Could I delete it? and would it delete the conversation in my old acc?
MacBook Air 13″
You can delete anything you want to; it's your machine.
If you have messages you no longer want to keep, then delete them.
My experience with messages is deletion on one device does not delete them on others.