Hi there @mostlyalex,
The conversion period begins when an email is received. The conversion data of your 30 day flow test should be visible on your account I believe.
When a flow message is deleted so are the analytics that were tied to that message. We suggest putting the flow into 'draft' mode. Then when you go to view your Flows, filter the status by 'live' so that you only see your live flows but keep the historical data of 'draft' flows.
If you are deleting part of a flow (ie. conditional split or a particular message) and wanting to keep the analytics - you could always clone the flow keeping the old flow the way it is with the analytics (just be sure that the old flow is in draft mode)and then delete the conditional split /message in the cloned flow. This article will give detailed instructions on how to clone the flow: How to Clone a Flow
Another option that you could keep the analytics of your old flow before you delete anything could be to run a flow performance report on that particular flow and group by those specific email messages, and then export the data. This way you will have the data of the messages and can edit the flow as needed. To accomplish this please follow the detailed instructions within this article: How to Build a Flows Performance Report.
Thanks,
Alex