I have done this on dual air Revs which was very straightforward but now need to do this on single air.
Is it possible? Is it easy?

You need a new air shaft but otherwise it’s straightforward.

I’ve just changed travel on a solo reba. I needed a new solo air shaft only. Should be c£25 or so. Yes very easy assuming you have circlip pliers, suspension grease, and 15wt oil to hand.

Check on the sram fork manual that your chosen travel is within the rebound damper remit too as I noticed some cover x mm to y mm and some y -z mm. You will also need to add or subtract bottomless tokens.