As an example, You swap a 2-day trip that has days with a higher cumulative negative reserves available than a 4 day trip.

2- day trip
Day 1 15 required 4 available
day 2 10 required 4 available

4-day trip
Day 1 15 required 12 available
Day 2 14 required 10 available
Day 3 10 required 6 available
Day 4 12 required 8 available

The 2-day is a cumulative -17
The 4-day is a cumulative -15

The swap would go thru.

Or for a real world example. There is a 2-day in NYC ERB right now, starts on the 18th, commutable on both ends, 1 leg to SJU, 1 leg back. Cumulative -35 on reserves. You could swap a 3 or 4 day from many dates the rest of the month and it would go thru.