Once a Frame has been created, it is not possible to change the number of axes it contains, but it is possible to change the order in which these axes occur. To do so, an integer permutation array is filled with the numbers of the axes so as to specify the new order, e.g:
INTEGER PERM( 2 ) DATA PERM / 2, 1 /
In this case, the axes of a 2-dimensional Frame could be interchanged by passing this permutation array to the AST_PERMAXES function. That is, an () coordinate system would be changed into an () coordinate system by:
CALL AST_PERMAXES( FRAME, PERM, STATUS )
If the axes are permuted more than once, the effects are cumulative.
You are, of course, not restricted to Frames with only two axes.
AST A Library for Handling World Coordinate Systems in Astronomy