SmartEX and Hayatoku 21 Question

Good afternoon. I'm going to be traveling from Shin-Kobe to Tokyo from December 22-27, and would like to take advantage of the Hayatoku 21 RT discount ticket through SmartEX (which I have never used before). Because these are presumably busy days I would like to reserve as far in advance as possible, but I know Shinkansen reservations are only allowed 1 month in advance, from 10 AM.

My question is this: according to the SmartEX someone traveling on these dates could book a round-trip on November 22 for travel on December 22 at the discounted fare, then on November 27, change the return date to December 27 (when reservations for that date open up). Am I reading the rules correctly - there are no penalties for changing the return date? I could wait and do the original booking on November 27, but I would be worried about the Shin-Kobe to Tokyo leg on December 22 filling up at the preferred times. Thank you in advance for your assistance!

Dan

Dec 22-27 are not busy days. Dec 27 is a Thursday and will be a regular workday at most places.

Thank you. I was a bit concerned because of the school holidays and such, but you think I would be OK to wait until the 27th to book the entire ticket?