Re: Windows 10 mobile insider upgrade
Yes, you can. First time you upgrade to insider preview, you can anytime opt for the production.
As fas as the normal upgrade OTA is concerned, Microsoft will start pushing Win 10 to older phones sometime in late March. So I am expecting that Lumia 640 should be getting it in the 2nd-3rd week of April. It can be further delayed as well. No confirmation has been given by MS yet.