When I select an existing requester and click on the option "Convert to an Agent -> Full-time Agent" the page refreshes but the user is not converted.
I've tried this with the latest versions of Firefox and Chrome on MacOS 10.13.3 (17D47).
As discussed in the support ticket, you were unable to convert a requester to an agent due to the password policy that you have set up.
The password policy for agents was different from that of the requesters and when you tried converting the requester, the password of the requester did not match the password policy setup for agents.
In the future, if you face the same issue, please reset the password for the requester in compliance with the password policy setup for agents. This should let you convert a requester to an agent.
Feel free to write to us at email@example.com for any further clarification.