I was trying to find an answer today. I called customer care and they told the same.
One person can hold only one LTF card. If you want LIC Select, then it will not be LTF.
Or else, cancel existing wealth card and then apply for LIC Select.
There is no cooling off period, you can apply soon after you cancel. (But I think it is better to wait for full cancellation + CIBIL gets updated.)
Also, No upgrade possible from Wealth > LIC Select.