Un-installed, a collapsed lifter looks no different than any other lifter in the engine. It's ugly head only rears up when oil pressure, which is sufficient to pump up the good lifters, will not do so with the bad collapsed lifter. A new lifter can be placed into the bore. I've mixed and matched used lifters with used cams all my adult life, and have never wiped a single lobe as a result. But when I do this, I treat is as if it were a new cam and used a quality break-in lube on the lobe and lifter, and performed the usual cam break-in procedure that's used with brand new cams.