Be warned about all this talk of old speaker compatibility. It is very misleading. I was advised prior to Airplay 2 release about the functionality we were likely to get. It is very limited. Based on that advise I now have mixture of compatible (SONOS ONEs) and in compatible SONOS speakers (PLAY 3, PLAY:1s, PLAYBAR). Firstly if you have ONEs set as surrounds to a PLAYBAR they will not be compatible with Airplay2 as it is a closed group and the ONEs cannot be seen, this is because the PLAYBAR is the lead speaker in that set (not compatible). This negates Airplay 2 completely. (this is what I did in an attempt to provide access to my older speakers throughout the house). The only way I can bring that theatre set into play is by having an additional play ONE (thats 3 now) to drive the airplay2 commands. Also, 'accessing' older speakers by simply having a single SONOS ONE limits your entire network of speakers as it must be part of the group. For instance if room A has a compatible SONOS ONE and room B has an in compatible PLAY:1 you cannot play music in room B without playing in room A too (as it needs to be one group/room not two), which kind of ruins the notion of multi room. Essentially you have to set up your entire multi room house as one single room/group and toggle the mute/volume on each room depending on which you want to 'hear'. This will then further restrict the beauty of airplay 2 and Siri as you will not be able to send room commands to seperate spaces. You will not be able to say 'play music in the kitchen' as the kitchen will be grouped with the rest of the house. Its a complete hack.
The ONLY way around this is to have at least one compatible speaker in each room/group. ie, ditch everything you have and start again.