You don't need to update everything lockstep. You maintain compatibility by converting the structs back and forth. However, all ambiguity is gone and everything is well understood.
This fails in several key spots. First, not every update should require a complete migration of all at rest or in flight data. Adding that as a requirement will instantly kill many efforts.
Second, you can't convert "to and from" a format you don't have specified yet. So, you have to first push out all code that can convert to a new version into all spots before you can start using that version. This will similarly kill many efforts.
If you are a smaller shop, you can jump both of those hurdles. But, I will gladly assert that extra work of jumping those hurdles will grow faster than your ability to advance past them.
