VERY unlikely third parties will be able to swap the hard drive itself. OWC had to stop doing that on 2011 iMacs because of hardware changes. And it would cost a ton at Apple (saw a post where it cost a guy just over $400 for a new 1TB).
If you're going iMac (and frankly I'm hesitant now that I can't replace my own drives) get the biggest you can now.
SSDs are possible, I believe, and you can ship the machine to OWC to have them do it.
(and then hope you don't get unlucky and have a hard drive failure AND have Apple tell you your warranty's voided 😱