Originally Posted by platbrownguy

Yes -- because you'll already have LAX-MIA in J, your reservation will have a "voluntary downgrade" to PE on MIA-SCL. There is no charge to upgrade that segment to J when U availability appears. You can also change dates and connection cities at no cost so long as you originate and terminate in the same cities.

Calling in to do the change is indeed very straightforward.

The only difficulty I might warn you about is married segment availability. There needs to be "U" availability for MIA-SCL (or wherever/XXX-SCL) not just on its own (i.e. not just when booking a one-segment itinerary) but as part of a complete itinerary. But don't worry about that unless and until it creeps up.