I have 10.7.5 and been meaning to do the upgrade to mavericks for a while, but nervous due to it being my work machine. Is the upgrade route safe and advisable for mavericks? I know some upgrades in the past (with mac and windows) would leave a bit of a mess behind due to slightly different structures. I'd prefer not to have to reinstall everything again, and not sure what to expect as app store only has an "upgrade" option. Is there any down side to doing the upgrade over a fresh install?