At this time, the ability to change an existing invoice back to an estimate is currently unavailable in QuickBooks Online (QBO). While this option isn't available yet, rest assured that I'll pass along your suggestion to our Product Developers. They're always looking for ideas to consider on how to improve QBO.
In the meantime, I recommend visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features.
Just in case, I'll add these articles for future reference: