Time Machine restore newer Mac to older Mac I will be replacing my current Mac with a newer model, at which point I'll restore my Time Machine backup to it, then wipe the old Mac. However, I considered that it might be a good idea to keep the old Mac in case something happens to the new Mac, but for that to be useful I would need to be able to restore the new Mac's Time Machine backup to the old Mac temporarily. 

Is it possible to restore a newer model Mac's Time Machine backup to an older model Mac with the same OS version? What about different Mac OS versions?
Yes, depending on the options you select when doing the migration.

In terms of documents and other data this will usually be fine. I'll provide an example below of when documents/data may be a problem.

In terms of software, this will depend on its compatibility with the version of MacOS you're running. For example, if you're running XYZ software that is only compatible with macOS Sierra and then restore that to a Mac running El Capitan, it will be copied across but won't run. Another potential issue is that you may need to re-register some 3rd party software (but if so, this is a minor inconvenience and does not result in any increased expense).

Now for my example of when data may be an issue. Your data will be copied across fine, but if the data is from a newer version of software that has actually changed the format of its data, then accessing that data from an older version of the software may be a problem. In any case, the data will still be there and is backed up.

On a final note, you would normally ensure the other Mac has an OS already installed when you restore your applications, documents etc.