Big Papa, I upgraded to W10 on my Lumia 640 and everything works just fine , I do realize many others have not been so fortunate but honestly for me all i did was plug in my phone to the charger, select the FAST option on the preview build and then waited......About 10-15 minutes later my phone was ready. BEFORE I really did anything I manually restarted the phone and then proceeded to use it as normal.
So far so good, the Lumia 640 is my daily driver, the ONLY thing that won't work and I noticed this today was that a friend texted me a contact to another friend and it was in a .x-vcard format (he sent it from his iPhone) and my Lumia could not read it, asked me if i wanted to look up an app for it on the Windows Store (there was non), so be aware of this.
There is a work around to this of sorts, just import the contacts to google and you can view them there, unfortunately texting contact info is not yet supported so far as I found out today.