Differences between V0.0 and V0.1?

[Shroooot]

What are the differences between V0 and V0.1? I see kits around for V0 but not many for V0.1 - does it matter?

[Demosth]

Differences of Changes in the V0.0 vs. V0.1
"I've already sourced all my V0.0 parts and/or got some janky kit! Should I just build a V0.0 then upgrade? What do I need to change to build a V0.1?"

It's OK to just build a V0.0 if you want. But if you wish to a V0.1, it's probably easier to start with a V0.1, to begin with. This outlines the changes. For each part mentioned, take a look at the sourcing guide and V0.1 manual for exact and up-to-date part numbers, sources, and other details. There is also a pinned discord message with the changelog and V0.1 release notes. This doc is intended as an accessible outline for those who already have a V0.0 kit and is not a substitute for other official documentation.

Essential parts to the source are highlighted. Whether something is optional or required is called out in bold.

Major new features
The most significant changes are to the Z-axis and the new tool head and extruder. Most of the other changes are optional because they just simplify assembly and/or won't significantly benefit someone who already has a v0.0. For someone who's just starting their build, the optional changes are worthwhile.

Misc
There are a few changes for fasteners. Take a look at the sourcing guide and see what's required compared to what you have on hand. In particular, the number and sizes of M2 and M2.5 screws, the addition of M3 lock nuts, and the addition of M2 flat head screws. Yes, you need flat-head screws. All the fasteners are obviously required for the new features.

The C13 power cord with an inline switch has been replaced with an inlet with a switch, but this change is optional.

The 16T idlers are replaced by stacks of additional F623-RS bearings. The 16T idlers suck and tend to blow up; the design was changed accordingly, so this change is required.

Printed parts
Many printed parts change and are required. Many of these are detailed in the V0.1 release notes. 

Panels
All of the top-hat windows change, and this is required to use the new tool head and extruder with the top-hat. However, if you decide to use the older bowden tool head, you can use the old top hat. There are also mods to extend the old top hat.

Other panel changes are optional. 

• The front deck panel is unchanged, but the back deck panel goes away. 
• The mid panel and back panel have changed, but you can use your v0.0 versions (they'll just have a few extra holes). 
• There is a new motor panel, but you can print that later. 
• The side windows and door are unchanged.

Z axis change
This is one of the most significant changes in V0.1. The Z-axis now uses a new Nema 17 motor with an integrated lead screw. This should significantly reduce the likelihood of Z wobble and dramatically simplifies construction. It's pretty central to the overall V0.1 build, so it is required unless you've already built your v0.0 and don't have any Z wobble.

Toolhead and extruder
With features like input shaper and a light tool head, the advantages of a Bowden setup are diminished. The new direct-feed tool head uses a new round pancake motor with the spur gear. The required BMG extruder components are the same. The part cooling fans are the same, but you'll need a new 30x30x7mm axial fan for the hotend. This is another significant change and strongly recommended, but you can use the old bowden tool head and pocketwatch extruder if you really want.

A v6 hotend is no longer supported for the direct-feed tool head. If you have a v6, you will have to replace it with a Phaetus/Triangelabs Dragon or Dragonfly, or a Slice Mosquito. The bowden setup still supports a v6 hotend.

Pocketwatch change
If you are still planning on using the bowden setup, the same new motor is specced for the pocketwatch to be consistent with the new tool head. Still, that change is totally optional and obviously not relevant if using the new toolhead.

A and B motor change
There are new, longer motors for A and B with more torque. These are very optional and are only worth changing if you're going for benchy races. There will be no quality change for most people.

Bed heater and PSU
The specced bed-heater has changed to a 60W, 24V DC bed heater. The old heater was overpowered, and this simplifies wiring a bit (and gets rid of the SSR). 

It also requires a bigger 24V 150W power supply since the bed is powered by the 24V supply instead of directly off mains. It's an optional but recommended change; you can still use the old heater and wiring for that, and if you do, you can use the old PSU as well. There won't be any performance difference. You'll need to reduce the duty cycle of a 100W bed to 60% in the Klipper firmware.