Hey Leo,
I'm not familiar with why a fund would so alter its cost basis, so I can't comment on whether adjusting the basis makes sense. But to do what you asked to:
Postings are adding or subtracting from an existing position. So adding or removing 0 units doesn't make sense.
Try a transaction which removes the old lot, and adds the new one like so:
2020-07-01 * "Amortization SPTW11"
Assets:Investiments:Broker:SPTW11 -208 SPTW11 {2019-12-31} # select the lot however you want (by cost, by date, or by label)
Assets:Investiments:Broker:SPTW11 208 SPTW11 {{669.76 USD}}
Income:Investment:RealizedPnL:Broker:SPTW11 # note that all transactions must balance to 0. This posting would contain the 14301.32 - 669.76 difference in value. Post it to a different account if Income:PnL is wrong.