XG18 Upgrade delete config, need Firmware 17.5.MR9 or a working XG18 ?

As i understand, you can't upgrade 17.5.MR10 to XG18.

I am only seeing 17.5.MR10 available now under software or firmware downloads.

I got some Information from sophos.

MR10 is only for the small appliances with low Memory.
MR10 is not meant to be used in the bigger models.

And yes, you can upgrade if you upload and boot the XG18 Firmware.
But XG18 doesn´t know About MR10. So XG18 deletes the config and load the Default XG18 config.

The next GA Release of V18 will resolve this issue. 

See: https://community.sophos.com/products/xg-firewall/b/blog/posts/xg-firewall-v18-ga_2d00_build339-is-now-available