6.2 or 5.6 agents should be able to connect to 7.0 collectors but I would recommend upgrading the agents as well. Is there a particular reason you guys have not done that?
Here's the compatibility matrix you can refer to for future use as well -> https://www.dynatrace.com/support/doc/appmon/insta...
Yes, the upgrade is free. But you will also have to upgrade your new license as well. Go to eservices (https://eservices.dynatrace.com/eservices/start.jsf) and go to your licensing. You can either get a temporary license for 7.0 to make sure the upgrade went well or you can get a permanent one.