Correct, the previous counteroffer is replaced by the newly proposed one.
From the rules, pg6:
15.3 Counteroffers are placed faceup on the board on top of any
other faceup cards in the “Counteroffer” box.
15.4 When proposing a counteroffer, the countering player
must discard a number of cards from his hand equal to
the number of faceup cards in the “Counteroffer” box (i.e.,
the first countering player does not discard any cards, the
second countering player must discard one card, etc.).
- When a new counteroffer is proposed, the previous
counteroffer is no longer the counteroffer and
the previous countering player is no longer the
15.5 Only the topmost card in the “Counteroffer” box is the
counteroffer. Previous counteroffers are ignored except
for determining how many cards a player must discard to
propose a new counteroffer.