If I’m holding an old-series derivatives position (in the original lot size) and the exchange revises the lot size, can I close my position by placing the opposite trade in the new lot size?

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No. Once the lot size changes, your existing “old-series” contract remains unchanged—and any sell or buy you place after the change date is treated as a new contract in the revised lot size. Your original position stays open until expiry, and you end up with two separate legs whose difference is your net exposure.

• Old contract (pre-change)
  • Lot size: 30 units
  • Your position: +30 units
• New contract (post-change)
  • Lot size: 35 units
  • Your opposite trade: –35 units
• Net open position:
  –35 + 30 = –5 units (you’re net short 5 units)

Opposite trades executed after the lot-size change do not cancel out your old-series position. They create a brand-new contract in the revised size, leaving the original position intact. To fully exit your old-series lot, you must wait till expiry or close it before the exchange revises the lot size.