Can in wall speakers be used in ceiling?

You might check to see if there is more information in the user manual from Sonos Support or if there is something from a similar speaker on the Sonance site.

https://www.sonos.com/pdfs/productguides/en-us/sonos_in_wall_by_sonance.pdf

Unfortunately it only speaks to the grilles and doesn’t mention if the speaker itself could be used in ceiling

In general, there is not much difference. A given speaker might project a pattern that is more appropriate  in a given situation, ceiling or wall. Ceiling speakers are more likely to have a tweeter that can be slightly aimed. Ceiling speakers often mount the woofer and tweeter on the same axis while wall speakers mount them beside each other. You can find discussions claiming that same axis is better.

Visually, most people feel that ceiling should be round and wall should be rectangular.

Some speakers will ship with both round a square grilles.

Do you know if the in ceiling speakers work with the rectangular grilles?

Depends on the mfr/model.

If you have rectangular speakers in the ceiling right now, the tweeter is probably separate from the woofer, so the use of inwall speakers on the ceiling should work for needs. 

My main worry is the speaker retention methods, what is adequate for a vertical mount may not provide the needed support for a horizontal mount.