Modify function only works for your first chunk when you drop the back half of your dates. What you need to do is then quickly make a new reservation for the second chunk. You should be able to make this change in 2 - 3 minutes. Sometimes if you do this too quickly, the dropped dates may not appear when you search for the dropped dates to make a new (second) reservation. That has happened to me a couple of times so no need to panic...just search again and you should see the dropped dates load again. To be safe, I try to do this late at night on the West Coast when fewer people are searching, but I have done it in the middle of the day when I was trying to piece together a reservation for renters with no issues.
What I suggest is that you practice with the Modify function by making a fake reservation far out, then try dropping dates, making a new reservation with the dropped dates, or try replacing your fake reservation with a reservation at another resort and go through the steps just so that you know what to expect when you are ready to modify a real reservation. After your practice session, remember to cancel all your reservations and your points will be returned intact to your account.
Once you get the hang of it, you can use the Modify function to re-allocate points yourself. For example, you have 2 reservations--one in March using banked points and one in June using regular Use Year points. Unfortunately you had to cancel your March trip and now you wish your June trip was using the banked points instead so that the regular Use Year points can be banked to next Use Year. All you need to do is Modify your June reservation, put in the exact same dates as your June reservation--don't change a thing. The system will show that your room is available since it is your room. Select the room and then go through the steps to finish the reservation. The system will automatically prioritize banked and borrowed points if they are available in your account, so your June reservation is now using up all your banked points before dipping into your regular UY points. Voila! You have just freed up regular UY points that you can now bank if you are still under the banking deadline or if you have passed the deadline, you can transfer the points to another account/member.