The available routes for bringing a project down
No single route suits every situation. Here they are side by side so you can pick by your own constraint.
Have the other side upgrade
Zero cost and zero risk, provided you can change the software version on the other end. Works when you control delivery and they are willing. It falls apart the moment the recipient is a client or a locked-down lab machine.
Rebuild by hand
Export the essentials to an intermediate format and rebuild in the older version. Maximum control and no compatibility surprises, at an enormous time cost — only worth it for very short projects.
Install the older version and re-save
Sounds direct: install the target version, open through a compatibility mode, save again. In practice the older version often cannot open the newer project at all — which is the problem you started with.
Use an online downgrading service
Hand the project to a tool built for version conversion and get back a file the older software opens directly. It saves installing old software and suits the last step before delivery. The thing to check with any such service is where your file goes and what happens to it afterwards.