If it's getting an error then you probably should revert to previous OS then upgrade again. If it's just not getting the upgrade ... it may not be it's turn. I have 3 with reservations, so far only one has gotten the offer.
It should simply apply the 10240 build ontop of whatever build you had no matter how old. Hopefully. And yes, expect it to take a while. Being able to download the OS sure is handy, but not so great when you consider the number of people around the world all downloading it at the same time (supposedly windows 10 is responsible for a 35% increase in internet traffic..)
I've had the 10074 and the 10158 Tech Preview running on a few systems without connected microsoft insider account.
After the DreamSpark iso's released i simply downloaded that, mounted it, and made an update. No Data loss
Didn't even ask for a Windows Key after i left the insider Preview, still activated win10 pro -.o