You can set up a calculation field with the If or Case functions to do this.
Set them to reference a global field or global variable in their boolean expressions in order to refer to different fields:
If ( Globals::gFieldSelector = 1 ; Field1 ; Field2 )
Case ( Globals::gfieldSelector = 1 ; field1 ;
Globals::gFieldSelector = 2 ; field2 ;
Globals::gFieldSelector = 3 ; Field 3 )
The Choose function can also be used.
You can also put the "table::name" of a field into a field or variable and use the getField function to access the contents of the named field.