They will continue to work as surround speakers for the Arc. That upgrade is what I did as well. 

Just make sure that before you pull the PLAYBAR out of service that you ‘un-bond’ both the surrounds and Sub, if you have one. They’ll be in separate rooms temporarily, but as soon as you get the Arc set up, you can then ‘re-bond’ them in their correct configuration. 

Thanks - I don’t know enough about Dolby atmos to know if it matters what surrounds I have …

The Play:1s will support Dolby Atmos when used as surrounds with an Arc.

However, to get Dolby Atmos your television will need to have eARC, otherwise you could use HDfury Arcana to obtain Dolby Atmos from external devices.

Thankyou. Upgrading the tv to one with an eARC HDMI socket is also part of the plan but may not happen at the same time!  Just want to future proof myself. 

Having eARC on your TV is only necessary if you want Atmos from external sources (like a BD-player). For Atmos from apps on a TV (that send compressed Atmos via DD+) HDMI-ARC is sufficient, providing the TV will supply Atmos to the Sonos soundbar.

When buying a new TV, buy one equipped with eARC to be future proof.