Just use your present 1.8-installation to finish your games.
1.9 will install into a different folder, so both versions can exist side by side.
You need to redownload your maps for playing with 1.9, using the ingame-downloader.
Also in case you have used a 1.9-prerelease version it might be that the maps you have downloaded are no longer working with the latest release (as there have been changes during the development process). In this case delete all files present in your
…\users"name"\triplea\downloadedMaps
directory and redownload the maps again.
Be sure to have the latest Java ( from java.com ) installed before (always a good idea).