Is it possibe to change a full wallet into a lite wallet?
yeah, delete all folder or rename it , the one contains all info and install new wallet "light" wallet this time.
It's not possible to change a full wallet into a light wallet (not in a straight forward way). Please don't delete anything unless you have a full backup (necessary to restore blackbytes) or the seed (enough to restore bytes).
What you can do is install a new light wallet elsewhere, then transfer your funds to it. Mind that by doing so, you'll have to link your new addresses.
Exactly what i said. if he rename it he can get access to it later. back up of whole wallet only possible with full wallet.
So one option is to install brand new wallet and move coins to there.
I wouldn't call it
exactly 
. It is very important to stress out that a wallet should be backed up before anything is done, especially when you tell someone to delete their folder ... Renaming is okay but this won't help them transfer their funds if they have any. Running two wallets is a must, and I think this wasn't clear in your comment.
Regarding this:
back up of whole wallet only possible with full wallet.
The full backup option is available for the light wallet also. Recovering from the seed is however only possible from a full wallet (maybe that's what you meant).