Full disclosure: I work at Braintree. If you have any further questions, feel free to contact support.
I think this question might be better addressed to our support team as there might be more things to work out with your integration and testing with regards to managing Braintree's recurring billing system.
Depending on your settings, events like changing the price of an active subscription can cause transactions to be issued immediately, or just impact the subscription's balance which can be determined by assessing the balance parameter of the subscription. This would happen against a subscription which was created on the same day as well, so that shouldn't impact your testing specifically, save receiving a webhook on its future billing date. Those webhooks however would reflect the result of your subscription billing the addition/difference between the next_billing_period_amount and the balance.
In either regard I'd recommend opening a ticket with our support team to work this out further.