The real reason why banks change currency often is to prevent good, honest people from time travel into the past.
If you could go back in time to buy something and then bring it back to the future to sell it as something immensely valuable, you’d need their currency. Of course, the only problem with that would be that your current currency would’ve changed!
Unless, of course, you travel progressively back in time, to a time where both your new currency and the previous unit was usable. For example, I could change my RM50 notes into older RM5 notes (right around the time of Chinese New Year, where such behavior is acceptable), then travel back more to change it to RM100 notes, and travel again to the point before it changed look to change it to RM20 notes, and so on and so forth.
At some point you’d want to change your money into tin ore, since that probably is the only thing that depreciated over the years.
Of course, the smarter of you might travel forward in time and sell what would be worth a lot in the future. Though the question is, what would you bring back in time, then, that would be worth a lot more?