Hello,
Getting totally rid of EV means indeed exporting 20000 archives back into Exchange.
Disabling all users, zapping all users, uninstalling EV.
Why do they want to get rid of EV? (personal interest question)
I would contact support. It should be possible to keep an EV-server running that acts as a ' shopping' server, which gives users access to archived mails, without the need for archiving. You could disable all users from archiving.
There might be tools available that can export the content of the archive back in the mailbox (in bulk), but do take in account the amount of space you need in Exchange! Check evtools.net (I'm not affiliated in anyway with them), maybe they can help.
Gertjan
Regards. Gertjan