Do I need to uninstall my ibis 99 or will this new version automatically update/overwrite the older version?
You do not need to uninstall your old copy of IBIS. The new installation should not overwrite your old IBIS.
However, to be safe, if you have added notes, comments, links, or patient records you will want to save you old "User" folder because such data is in that folder.
A new updater/transfer tool will be available in the near future to enable you to import your old customizations.