I want to switch my number to Verizon, but want to keep my old AT&T plan. Can you do this?

Is your son's line currently on your account or is your number the only one? If yours is the only line, the account will be closed when you port out the number.  Is the plan you currently have a grandfathered (no longer available) plan? The only way to maintain a grandfathered plan is if it's on a line that's already active. In other words, if your son already has a line on your account and the grandfathered plan is already provisioned on his line. If it's a family plan however, once your line is ported out, an individual plan will need to be provisioned, which will eliminate the grandfathered plan. 

 

I hope this answers your question! 

Usually not, especially if the plan is no longer available. Once you port out your number, the account is automatically closed.

the only way to keep the grandfathered features would be to keep the account under your name.
Even if you do a Transfer of Responsibility into your son's name, it'll remove all of the old / expired features.