Are Makerbot Smart Extruder nozzles swappable? I'm wondering if anyone has any experience with Makerbot smart extruders and knows whether or not the nozzle size can be changed. I'm trying to figure out if I can use different nozzle diameters (0.25mm, 0.5mm, 0.8mm, etc.). 

Makerbot doesn't sell any replacement nozzles on their site, so I suppose I'm just trying to figure out if the nozzles on the makerbot use the same diameter/thread pitch as other nozzles, like the ones from E3D or if I'd have to mill my own, which is way more trouble than it's worth. 

Any research on the subject is pretty sparse, and the best resource I found is a blog post from 2011 when they still allowed their customers to do such things themselves. I've searched around their Google Group but got nowhere. Beside that, there isn't really any information from the last three years or so on the Makerbot machines.
Yes, you can change the nozzles with the 5th gen line printers, although it's not recommended or supported by Makerbot. You're on your own if something goes wrong with an aftermarket nozzle. Here's a swap video from Fargo3D: https://youtu.be/vL80bslk9vw

I would recommend "mk8" Makerbot Replicator 1/2/2x style nozzles, since these will be similar dimensions to the original nozzle. Ebay has lots of cheap ones, or P3-d and Micro Swiss are popular options for premium Makerbot mk8 style nozzles. But you should be able to use any standard M6 male thread, ~2mm ID nozzle (such as from E3D, because the Smart Extruder Z homing routine will compensate for nozzle length. 

When you change the nozzle, you're also going to need to change the slicer settings. Do not use default settings with a different nozzle. Smaller nozzles will require significantly lower print speeds, and larger nozzles will require wider extrusion width. You can create a custom profile in Makerbot Desktop to do this, or use a 5th-gen-compatible slicer like Simplify3D.