Hire counsel. Sounds like the lien was released and their is no claim to the vehicle, but it also sounds like the new conservator is going to need some convincing. Don't try to do it on your own. Do not delay as the passage of time will negatively affect your rights.
Wait a minute. You as conservator for your Dad released the lien on YOUR car? Unless you can show you actually paid your father's estate the obligation that lien secured, then you have engaged in self-dealing. No wonder the new conservator wants the car. Mr. Schwartz is correct that retaining counsel is appropriate, but be aware that the new conservator may well be right.
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
I agree that you should consult with a lawyer and do so without delay. That will give you opportunity to share details of the transaction, details that you should not post on this public message board. Your lawyer will need to analyze the facts and circumstances of the transaction to provide you with an answer regarding the vehicle; and based upon what you've stated in your post, the answer is definitely fact dependent.
For example, if you acted as conservator and transferred title for either no or insufficient consideration, and the vehicle is community property, then you may have engaged in self-dealing. If the vehicle was not community property then you may or may not have engaged in self-dealing, but you may have not upheld your fiduciary duties to your father as his conservator and guardian.
If your father assigned you additional rights in the vehicle, then the answer could be different from the other scenarios. The bottom line is that you need to hire counsel to provide you with guidance based upon your past behaviors and the facts and circumstances surrounding them. Until you know whether or not your actions were legal, you will not know how to proceed.
Good luck in working with a lawyer to figure this situation out and in proceeding with the utmost integrity.