for now, when changing a subscription offline (without using the customer portal), there is no option to modify the date the changement should start. It's necessarily the today's date while, when you are a reseller of a solution, you may need to invoice after technical changements have been done or are planed on a future date.
Thus, we need to be able to change the modifying date the same way we can set a start date in the past or in the future when creating a new subscription.
Thanks for sharing your requirements. I'm adding this as a feature request for our team to consider for future revamps and we'll make a note of general interest for having a custom effective from date for updating subscriptions and prioritize the feature accordingly.
Let me know if you have any questions.