2nd Time Aortic Valve Replacement

My Mother is presently 57 years old, underwent Aortic Valve Replacement in 1994 with (No.20 Medtronic Hall Valve in Aortic Position).
So far she was normal, and last month her echo showed Aortic Stenosis with aortic regurgitation - valvular(stage 1).She has symptoms of tiredness and headache and otherwise she is ok.
She was having regular checkup with the same Cardiac surgeon who did the surgery in 1994. The cardiac surgeon suggested to wait for some more time and if the conditions doesnt improve, my mother has to undergo another surgery for replacing her aortic valve.  
1. Please advise if the condition improves, is the surgery not required.
2. Please also advise will the suregery be required in future even if she is improving now. If so what would be the ideal time to undergo surgery.
3. Please advise which mechanical valve would have higher lifetime , so that she neednot undergo another surgery.

Thanks in advance.
Regards
T.Srinivasan