When the spell ends, the target can't move or take actions until after its next turn, as a wave of lethargy sweeps over it.
However, let's say Character A cast Haste on Character B. In the 10th round (final round of haste if concentration not broken), Character B casts Haste on himself.
What happens during the next round?
- Do they lose a round of the "new" haste while they are lethargic and lose movement and actions?
- Does the new haste override the lethargy until it ends?
- Is it some combination? Retain AC/Dex modifications but lose actions/movement?
- None of the above?