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.