delete() and new() on elements of an array created by new[]()

This may be a very dumb question, but brain can't seem to find
anything to confirm/deny suspicion.

I have an array created with:
   FooClass* foo = new FooClass[numberOfFooObjects];

I want to be able to replace an element of that array at will with a
new FooClass.
Am I right in thinking that to delete foo[3], say, and replace with a
new FooClass, would require placement new? Would this be wise?

It seems that if I want to be able to replace elements at will with
newly constructed elements (rather than using the assignment
operator), that it would be better to do this:
   FooClass** foo = new FooClass*[numberOfFooObjects];
Then create each object on the heap and store the pointers instead.
Replacing elements is easy and obvious then...

I feel I'm being really stupid here, can someone enlighten a poor
confuzzled coder please?
TIA :-)
--rob