ECVs retain their excess value until used or expired, so in that case you would get miles and retain the balance for another ticket. Most of the free ticket vouchers have a value of $250 towards ticket(s) and earn miles when used that way, except for those nasty ones that book into K and earn no miles and supposedly aren't upgradeable.
I never have questioned another post but in this case I think a correction is in order.
"K" fare tickets do earn miles + elite bonus miles and status miles. I can also assure you a platinum member gets complimentary upgrades, if avail, on these fares. Probably Gold and Silver as well.
I'm not sure but I think the excess ECV value needs to be used by the second ticket. For example, $500 ECV buys $200 tickets has $300 left. If the next ticket is less than $300 the remaining is lost. I've never had this happen but I seem to remember reading that rule.. I surely could be wrong on this one.