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 was fixed in OBAS 4.5.7 release.
To resolve the issue, please upgrade to 4.5.7 as described in How to upgrade OBAS system to the latest available version.
Or, alternatively, check actual billing periods configured for hosting plans involved before using the Prorated based on the period option.