That wasn't fun.
Having just designed several parts which go together for a prototype, I was anxious to get them all printed for a test fitting. Unfortunately, I tried over and over again with the bottom to get it to just print. And time and time again, the first couple of layers were being really inconsistent.
My initial three attempts were using a brand new Shaxon Natural PLA so I had no experience with this. It just wasn't adhering and it wasn't very consistent. I switched to a brand I knew and kept going.
This one was failing, too. Okay, so maybe it's the new relative extrusion setting in Cura (or the new version of Cura itself)? I sliced again with absolute extrusion. Still no good.
I thought I had one that was working and went grocery shopping. I came back to see it "air printing" a few layers above the part. I noted zones where it just wasn't printing at all and then "played catchup" where it pushed too much plastic. I cleared the clog and tried again. And again.
It finally dawned on me that after a year of printing (and a kilo of carbon fiber PLA run through it), it was time to change the nozzle. I did have the right parts but rather than risking breaking something in the hotend area, I bought some tools. Fortunately I keep printer spares for just such an emergency and I had the exact type of nozzle.
I moved the assembly to the front of the printer, retracted the filament and next heated the extruder to 265 degrees C. I carefully observed where the hidden (fragile) connections were in the back before using any tools. Once the extruder was at temperature, I carefully turned the nozzle counter-clockwise to break it free. I then used a pair of needle-nosed pliers to slowly remove it, catching it in a porcelain cup. Reversing this with the new nozzle, I carefully tightened it down under heat and then cooled the extruder. (In hindsight, I would have liked to have turned off the stepper motors but I was afraid if this would also turn off the hotend, too.) I homed all the steppers before continuing.
That was it. It's now printing beautifully. It was unnecessary to fine-tune the Z offset since it was the exact nozzle as stock.