Can I still edit a tour after handing it over?

Yes, if you chose Administrator or Editor under What you keep on the tour afterwards when you made the offer. What each role can open is set by the owner's Client Workspaces, which after the handover are the client's. By default an Editor can change the tour's panoramas and hotspots, and an Administrator can also open Design, Settings and Publish, but only the owner, who is now the client, changes who can open the published tour or puts it in a Vault. If you chose Viewer you can look at the tour and its statistics but not change it. The client can change your role or remove you at any time, under Project Collaborators on the tour's Settings tab (in the Advanced view). If you chose Nothing: it leaves your account, you cannot open the tour afterwards.

The panoramas of the tour count in the client's account from then on, not in yours.