If you switch to an annual subscription in the middle of your subscription, the remainder of your payment will be discounted from the first annual subscription payment.
Example
CMS Monthly Plan 2,480 yen
CMS Annual Plan 29,760 yen
Sign up for a monthly plan on May 11, 2023 → 2,480 yen debit on May 11, 2023
Renew monthly plan on 06/11/2023 → 2,480 yen debit on 06/11/2023
Change to an annual plan on 25/06/2023 → 28,520 yen debit on 25/06/2023
Fee per contract period
2023/05/11 - 2023/06/10: 2,480 yen
2023/06/11 - 2023/07/10: 2,480 yen
2023/06/25 - 2024/06/24: 28,520 yen (29,760 yen - 640 yen)
The ¥28,520 debited on 6/25/2020 is discounted from the CMS annual plan of ¥29,760 by ¥1,240 (exactly half of the ¥2,480 BASIC monthly plan) that has not yet been used.