In piano scores since (as best as I can tell) Robert Schumann, "Verschieben" is used to refer to the shifting caused by the una corda pedal. Usually it is written as "mit Verschieben" which is literally "with shift" meaning you shift the keyboard with the una corda pedal--as opposed to the English convention where we emphasize that we're "pressing the pedal" which in turn shifts the keyboard. I'm not altogether sure why those terminologies and uses developed separately. It is "mit Verschieben" on page 21 where no other pedal instructions are going on.
The composer is being extremely specific about using both pedals in this piece. So, at the bottom of page 13, you're instructed "wieder beide Pedale" which is "continue [pattern] with both pedals" meaning--as best as I can tell--una corda and sustain pedal continue in the same pattern until the next instruction.
When you hit "nur Verschieben" it means that you are only following pedal instructions with the una corda until you're told to use both pedals again (either by a "beide Pedale" or "Pedale" instruction). It seems that you're supposed to start half-pedaling on the next system with the "etwas Pedal dazu" meaning "bring some pedal back."
The following is largely a guess, because in any score engraving methodology there's never been full agreement in how to communicate multiple pedal instructions at once, so I speculate that they are using "Ped" and "*" and the "u.s.w." ("und so weiter" or "continue [the pattern] as such") to refer to when you lift or press any pedal in question. However, it is possible that the composer is wanting the shift to be continuous until you're told otherwise. I lean towards the former more strongly because of certain consistent patterns in the instructions, but that's not necessarily guaranteed. I'd recommend that you just experiment with both as options and see how playable either one is and how it sounds. Frankly, I don't like the una corda on grands very much, as it breaks my brain to having a constantly moving target as I play, so if I'm going to engage the una corda, I'm going to leave it there as much and as long as possible, so that I don't have to compensate every few seconds for the keys moved to... But, it seems that this piece is probably intended for frequent, constant shifting.
Also, based on the future instruction in piece IV of "viel Verschieben" I'm guessing that the frequent shifting could be meant to be a lighter half-pedal of the una corda through this Tango, rather than a complete shift at every pedal marker, but it could also be that in piece IV on page 19 may not be intended to help in interpreting the instructions in another piece in the same score. Hence, we're back to the fact that nobody ever standardized how to differentiate instructions for various pedals or changes in piano design. (Beethoven Sonatas, for example, can be a pedal minefield because the current reading of instructions is debatable when compared to how you'd follow the instructions on simpler fortepianos--which is itself a confusing and interchangeable term with pianoforte. )
Good luck with the piece! Gilles is right that this definitely involved material: Kudos for taking it on!
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