The following page still has references to MOD Team that need to be changed to Extensions Team:
https://www.phpbb.com/mods/

Also within that page the Junior Validators link leads to this page:
https://www.phpbb.com/mods/team-overview/?p=jr

The link needs to be changed to this one instead:
https://www.phpbb.com/extensions/junior-validators/

The old page with the form (the one under /mods/) can be done away with. On the Junior Validators page please remove the following text:

If you would like to become a Junior Validator, please read the requirements below, and if you meet them, you can apply to become a Junior Validator by contacting DavidIQ.

And add the following section to the end of the page:

Would you like to join the Junior Extensions Validator team?
If you would like to become a Junior Extensions Validator and meet the requirements above, you can request to join our junior team here. A member of the Extensions Team will contact you for further details or information.

The here should link to the following page:
https://www.phpbb.com/community/ucp.php?i=ucp_groups&mode=membership

This page allows the user to request membership to a group. There might be a way to either link directly to the group membership request by submitting the form from the Junior Validators page or through some additional querystring parameters. This part will need some investigating to properly implement.