I have a SuperFly 9.6 2015 which has a tapered “Trek E2” headtube, but the bike came with a Reba RL with a straight steerer.

I’m unclear as to what I’d have to get to put in a tapered steerer. Would this just be an IS bottom bearing and can I keep the rest of the headset, or is it zero stack and I’d need to swap the bottom assembly?

I’ve been trying to get me head ( 😆 ) round these new fangled things, but as you may be able to tell, I’m struggling…

I assume you have a ‘reducer’bottom semi-integrated headset to reduce the bearings from 1.5″ (tapered) down to 1 1/8″ (straight). So you should just need a normal 1.5″ bottom part of the headset (will need to knock out the existing bottom headset and press in the new one).
Probably best to replace the whole headset so you also get nice new upper bearings.

Thanks for the answer. I dismantled it this evening and it seems to have a reducer crown race on the fork but a 1.5 lower bearing/cup already in there.

I’ll get replacement bearings since it’s apart, but not touch the headset that’s pushed in there. I assume then all I need is an appropriate crown race for a tapered steerer fork.

Yeh. You need to get a crown race that matches the bearings. You will get a crown race included with a new headset. But it you just buy the bearings, they wont come with a crown race.
When buying the bearings, make sure you get the correct ones. Different bikes take different bearings (often the same inside & outside diameter, but different angles on the bearings). I recently replaced the bearings on my 2011 Trek Fuel, and it took me ages to confirm the bearings i needed. If you get lucky, you might be able to read the angles or part numbers from the existing bearings, but mine had all worn off.

If you get lucky, you might be able to read the angles or part numbers from the existing bearings

I got lucky 8) and also managed to figure out what crown race I need based on those angles – so unless I have muppeted when ordering, I should have all the bits coming shortly. I’m a bit of a classic car buff, so not entirely unfamiliar with having to source appropriate replacement bearings/races for things where the original part either no longer exists or is very hard to find. 🙂

fingers crossed for no muppets. 😆

good luck, and happy riding.