In order to delete a user, go to Settings > Users and click on "Manage User Permissions" and delete the user. When a user is deleted the campaigns that he/she created are transferred to another user.
If a user who connected the ad account gets deleted, that ad account needs to be re-connected.
In order to avoid that, you should transfer the ad account to another user before deleting the user. The new user that'll take on the account should follow the steps below:
Go to Assets > Accounts.
Reconnect the already connected account.
After transferring the accounts, one thing to keep an eye on is that if you see any problems with creative previews, is could be because of not having page access. And secondly, if you are unable to see which ad account is selected when you are on Information step of the campaign, that could be arising from not having access to that ad account. In this case, that ad account should be reconnected.