Hello flyfisher,
There are two areas for this question. One, assigning users to opportunities and lads. Two, having people view records that are assigned to other users. You should by default be able to see everyone's records. You can keep users who leave Active until you transfer items over to a new user. You license count is based on if the User can sign-in, not if ithe user is active, which is found in the first step of the user edit screeen.
First, lets look at assigning users to opportunities or leads. Each opportunity or lead is assigned to a Method user. You can manually change the user assigned to the opportunity or lead by going to the corresponding edit page. The only way to mass update the opportunities/leads is to do an import update. There are instructions here on how to do an import update. You would be changing the AssignedTo field and SalesRep field for Leads.
Second, there are permissions. You need to be able to share the records correctly. In the Edit User screen (the path is Customize -> Users -> Edit... -> Step 6 of 10:Access to Shared Records.) Here you can set how users share records. There is a blog post explaining how to setup sharing files amongst users. Could it be an issue with sharing that you are experiencing? Remember both users need to have the same permissions. (One must grant permissions to share and the other one must they will share with the first user)
Greg