Hello! I have been summoned by libra89!
I suffered a gravitational accident with my 640 in early 2016 or so (I just sold the 640 recently but that's besides the point) and ended up replacing the display.
The act of replacement itself was fairly easy and relatively cheap as far as phones go.
I spent about $20/25 on eBay getting a random 640 glass/display assembly (no frame or anything, just the panel and LCD) and I see them going for approximately $30 give or take on eBay. This was a relatively tough pill to swallow because the phone cost only $30 (AT&T) but lets be real, by itself that's a great price for a decent phone's replacement screen. (Had a Priv for seven months and if I wanted to replace the screen + glass, it would've set me back over $200 in parts!)
I found replacement instructions online (it wasn't this specifically but it the method was seen here
https://www.youtube.com/watch?v=ikFOuyumrNU - I'd find a better source though) and it went smoothly except I didn't have anything to hold the new display in place, having wrecked the adhesive when I took it apart the first time.
I highly recommend buying a little bit of those thin, black double-sided tape rolls (should be cheap) and lining the frame with it when placing the display back on. I used superglue and it worked but now it's never coming off again and it fogged up the front camera a bit.