On hosting plan change Prorated based on the period option just offers every single period configured under
Top > Billing Director > Settings > Subscription Periods, no matter what periods are configured in target or source hosting plans. The same is for the account change.
Such behavior was classified as the software issue PBAS-30321 and will be fixed in future OBAS releases.
Please check actual billing periods configured for hosting plans involved before using the Prorated based on the period option.