well, as explained in detail in How to update your Elive, the correct way - #15 by Thanatermesis , you should read the changelogs of the releases to know what has changed and then you can see what things you have "updated" with simple apug or other things
for example from 3.7.5 to 3.7.6 basically you can apug and install e17-menus and you are done, from 3.7.4 i dont remember now 
in short, apug only update packages with improvements but on the case 3.7.5 -> 3.7.6 it will not make your system to install e17-menus which you need, or if there's a default-conf improved you will not have it because confs are not restarted in every upgrade (install)
well, the good thing is that reinstalling is not a "big task" with the upgrade mode of the installer
Elive had in the past a very nice feature where you just needed to download the .iso and your system boots from that file just like a real usb/cdrom and you can reinstall from that (so you dont need to burn / record any usb/cdrom at all ) 
, could be nice to have that feature back 
, by other side, recording a usb is not a big task too (unlike the times where we needed to record DVD's or DVD-RW, which was also not a very reliable media)
