It will replace the old version. Worlds made in 1.6.4 could be affected in small ways, especially if you were using mods doesn't have a 1.7.10 version. I suggest making a second installation in another directory (minecraftedu2?) by downloading the new launcher from
minecraftedu.com instead of updating. This way you and your students can use whichever version you like. You can copy your worlds to the new version's directory too. Once you've concluded that everything is compatible, you can delete or rename the original installation.
Hope that helps.