Honestly I considered a clean install, but wanted to see if it would actually work. For me it worked better than expected. I have had good luck with Xubuntu LTS to LTS upgrades, and wanted to see if it would be the same with Ubuntu MATE LTS to LTS. Xubuntu probably takes around 2 hours to dist-upgrade. Ubuntu MATE took twice that long, which made me suspect it would not work. The only issue I had after the upgrade is a longer boot time. But my install was not complicated, hadn't been tweaked much or anything. Of course your mileage will vary according to your hardware and what worked surprisingly well for me may bork your install.