10sp Sram Red: red Indexer switch to 11sp Indexer?

[jemima]

Vaguely remember reading something about this here? ages ago, but wanting to confirm if the 11sp Red Indexer is a straight drop-in to 10sp Red shifter > can then use an 11sp cassette on a 11sp freehub.

May have also read there is room on the 10sp Red [red colour only?] Indexer to add an 11th 'notch'.
However, wouldn't adding that mean the extra cog [+spacer/s] on a 10sp cassette build would make the cassette width/fit too wide for an 11sp freehub?

[Xabi]

Ratio sells the kit, you can turn it to 12 or 13 speed too if you wish so.

I turned my 10 speed hydro levers to 12 speed eagle, runs smooth as silk with rotor round chainring, sunrace 11-50 cassette and ybn chain. SRAM eagle chain was a bit rough, probably because of the non SRAM cassette.