Is there any way to change the dimension of a field in a structure?

Hi,

      I've been trying to change the dimension of a field in a
structure, like the followings:

S = {Dog:lindgen(3,112), Cat:'cc', Horse:[1,2,4]}
S.Dog = lindgen(3,118)

I got this:
'Conflicting data structures: structure tag,<LONG      Array[3,
118]>.'

This is reasonable because IDL dose not allow to change the dimensions
of the fields in the structure.

I can solve this problem by redefining another structure S1 with the
same tag names of S:
S1 = {Dog:lindgen(3,118), Cat:'cc', Horse:[1,2,4]}
S = S1

However, the question is, if I only know the tag name of the field
Dog, and I don't know the tag names of other fields in S,  how can I
change  the dimension of Dog?

I've tried this:
aa = tag_names(S1)
S = {aa(0):lindgen(3,118), aa(1):S.(1),a a(2):S.(2)}

but this only got a syntax error !

Thanks!