Did you follow the instructions in this article?

https://support.sonos.com/en-us/article/downgrade-a-sonos-product-from-s2-to-s1

Yes I did. At some point it began to downgrade, but eventually failed. But more often it just failed to begin with. 

I would maybe give it another go from ‘factory reset’ using a reset S2 App, as they are S1 compatible according to this link:

https://support.sonos.com/en-us/article/sonos-s2-compatibility

If no joy after that then it maybe best to submit a Sonos system diagnostic report from within the Sonos App and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can perhaps suggest to resolve the matter.

Connect the IKEA speaker to your router then try to downgrade it. If successful, repeat for the others.

SOLVED! 
 

After gerning in contact with support I got the issue solved.

My guide:

1. I rebooted the router (don’t know if it is needed)
2. I connected the speaker with cable.
3. I deleted the S2-app and all data.
4. Reinstalled the app.
5. Resat the speaker (play/pause and + while power on).
6. Opened the S2-app and chose to downgrade.
7. Repeat for next speaker.

I found that there has to be a s1-Unit on the network too.

Thanks for reporting back here - that’s really helpful and glad you have got everything sorted.