It will find version 6 and remove then carry on with install. There is not a rollback facility and you need to install version 6 again after removing version 7.
If you have version 6 saved to a folder it will be a straightforward re-install otherwise you will need to download 6 again. Best way if 6 is not saved is to down load 6 first but save it to a folder or desktop but don't run it. Download 7 and give it a try then at least if you don't like it you can remove and reinstall 6 without going back on-line.
I tried a scan, took 17 minutes compared to 12 minutes with version 6 but it is far more detailed with lots of options that you can control. Future scans may be quicker?
They make their money off the versions for business and they hope you might go on to the Pro version. Personally I have used AVG for years and as I have said before, there is no point in a home user paying for an AV or a firewall, if they need one.
It also gives them enhanced market penetration without paying for advertising, marketing, storage and distribution. Grisoft and Alwil are commercial concerns and when the sums stop adding up, so will the freebies. Take them while you can.