The TXo+ is just a TXo with a more powerful processor (the Teensy 3.6) and a little daughterboard that makes the 3.6 happy with the ramp-up characteristics of common euro power supplies. The hardware underneath for this year’s TXo is identical. (More on this in a moment.) If you have a TXo all you need to do is slide off the processor and pop the new one one. That’s it. Done. Super-easy and beginner-friendly.
There will be pre-built TXo and TXo+ in the store. The upgrades are designed for previous TXo owners - though there is nothing to stop you from purchasing a TXo and an upgrade from this run. It would just cost a little bit more combined since you are buying two Teensy processors. I’m not doing any custom mix and match; no time for that with all of this shipping. 
About the Slight Difference (and the capacitors that shift with the upgrade)
TXo prior to this year have a slightly different cutoff for their single-pole, super-wimpy reconstruction filter. For this years models, all have a slightly higher cutoff on the filter to match the sampling rate when upgraded to a TXo+. For older TXo, the upgrade pack contains the capacitors that would allow you raise your cutoff the ~ 4 hHz that would be needed to make them identical. You don’t really need them unless you are feeling excessively anal.
I strongly recommend against worrying about it. The filter is super-weak and only has a very gradual cutoff as it was designed for filtering the CV, not audio. I honestly doubt you would hear the difference. It only affects the TXo when it is in oscillator mode - and then only slightly. That is why I recommend additional filtration if you want a cleaner sound from the oscillator.
