Man, I had a return customer that had one light with moisture in it. He parks his car outside.. I don't recommend anyone relies on only re-using stock sealant.
The guy that says "All you gotta do is push it back in the crack" is taking risk. Thank god the customer was driving a NISMO, only took 15min to get the bumper off and warm up the light..
If you do this stuff for other people always use new sealant:
OCI Butyl Rubber Glue - Accessories from The Retrofit Source Inc
$15.00 is good money for piece of mind...