Certainly migrating is something you want to do with ample preparation.. have you read the compilation of 7.0 to 7.1 documentation at this link? Jmart pulled together some excellent documentation links.
http://www.symantec.com/connect/node/1525181
Specifically, did you read the ITMS 7.0 to 7.1 Migration Guide?
Best practice is to use a new server name and new IP address for your 7.1 server so that you can continue to run 7.0. It sounds like your 7.0 server has been blasted away, so I would tackle this as though you've lost your old server.
First, confirm that your 7.1 server is healthy. Are logs relatively clean? Are packages replicating? Can you install the agent on a brand new computer without issues? If this is working, I would proceed to migrating clients.
If you use site servers, you will want to upgrade those. Best practice is to upgrade them to 7.1 in batches, as you move clients over. If you have just a single site server, you can migrate it to 7.1 or stand up a new site server and install the 7.1 components.
With your SMP working properly and your 7.1 site servers functional, you're ready to upgrade the clients.
The first step is to redirect the agents to the new server. It sounds like you used the existing server name, so they should already be talking to it. Working with a single agent (because you want to test things before you roll it to every agent in your environment), clone the Symantec Management Agent 7.1 non site server policy and then apply it to the single test computer. With it applied to your test computer, enable the policy and provide a schedule. Force an update of policies on your test client and you should see the agent versions update. It will close and relaunch the agent for you as it does this.
At this point your plug-in upgrades should still be disabled. Only after your agent is upgraded do you want to enable an upgrade of plug-ins to the 7.1 versions. Again, clone them and apply to the test again to confirm this works well.
When you're comfortable doing this for your test agent, expand to a pilot group; with this working well and without hiccups, enable the real policies for all agents and let them upgrade. Again, upgrade the SMA to 7.1 before you update the plug-ins. Don't upgrade more than a thousand nodes at once.
I can't find a reason that the 7.0 clients would not be communicating with the 7.1 server. How do you know they're not talking to the server? If you try to update the configuration on the client locally by clicking 'Update' on the SMA settings page, do they not receive updated configurations? What KBs have you followed or what steps have you tried? Are any agents communicating properly, or is it all agents that do not communicate? There are no port changes that would cause this.
There is no need to do an uninstall and reinstall. The Quick Run may have failed because it had dependencies on plug-ins that were not yet present. Was the error code -1? This typically means a plug-in or component is missing.
Is your NS properly processing logs? Check the \\NSservername\NSCap\ folders for .nse's. You shouldn't find many if your NS is working properly. NSE's should disappear quickly from the EvtInbox and you should not find a large number of NSE's in the Bad queues.
Have you reviewed the logs on your new NS? Is performance acceptable on your new NS, no CPU bottlenecks, etc?
If you can post replies to all of the questions I've asked, and try the steps I've recommended, you should be able to get your environment operational again. You should also read the Migration guide even though you've already migrated, because there are steps there that are necessary to follow for the migration you've just started.