Example
Do you have two existing documents — the first numbered VF1-0001/2021 and the second numbered VF1-0002/2021 — and need to swap them?
You always need to first change the initial document to a non-existing internal number value (e.g., change VF1-0001/2021 to XXX).
Only then proceed to change VF1-0002/2021 to VF1-0001/2021.
In the final step, change XXX back to VF1-0002/2021.
The above can be done via import (particularly useful when working with a large number of documents), or manually after adjusting your company settings (Other -> Allow manual override of internal document numbers; this option is best suited for a small number of documents).