I am brand new to triggers and am trying to accomplish a few things here since you cannot default the "Keep Account Team" checkbox on.
There is a checkbox that allows you to keep the account team when changing the owner as described here:
You might be remembering something similar to the idea out there requesting this be checked by default so account teams aren't accidentally lost when not checked:
When assigning an account, it is not possible to lock the account team: there is no control on the checkbox "keep account team", and no API access around assignments. (note: maybe one can create an Apex trigger for it, but it wouldn't change the UI of the assignment page)
Edit: Another idea here also states it is not possible to keep the account teams via API as you thought:
Allow Keep Account Teams and Keep Sales Team when changing owner via API. Essentially, if we change the owner via the API we don't have the ability to have the Keep Account Teams and Keep Sales Teams checkboxes checked like you would if you manually changed the owner via the UI.
Edit 2: After testing I also confirmed that the account team is kept when the account owner changes, in the workbench, in the developer console as well as during a jitterbit upload. I found a note at the bottom of the help sheet here that does make it sound like it's expected behavior.
After you change a record’s owner, the previous owner’s visibility reverts to the default sharing setting unless that user is on an account or opportunity team. For example, if the previous owner of an account is on an account team, that user has Read Only or the access specified in your organization-wide default for accounts, whichever is greater. The previous owner’s access to related records remain the same. This behavior is the same for opportunity owners that are members of an opportunity team as long as you change ownership using the API. When you change ownership of an opportunity within Salesforce, you can select the access level you want.
The checkbox option will appear once you click on Change Owner link on the account record's detail page. Please find the screenshot below:
Thanks!
Best Answer
We had a situation similar to this where users were not selecting the right boxes and selecting the wrong ones.
The solution I ended up doing was to remove the Account Owner field from the page layout and replaced with with a custom account owner field:
(Note: we opted not to care about the profile image, there are some tricks that can be done to facilitate this if you care).
Then I made a VF page that used the standard controller with a custom extension.
This allowed me to remove the unwanted checkboxes and customize the behavior completely. We did think about taking the approach that you are trying to take with using a trigger to correct the mistakes but we opted against this for usability; very confusing to a person when the click a box or don't click a box and see the opposite of what they thought would happen. We also wanted effectively non of the built in functionality to work and this was a much easier way to achieve that as opposed to writing a bunch of logic to undo each and every one of the checkboxes. There is also the risk that when the changes are being made manually (not necessarily by changing the owner) that your trigger jumps in inappropriately.