I've been wondering about this for some time. Is there any reason why you can't put roller lifters on a flat tappet cam?

 

the two cams are made of dissimulator metals ,and accelerated wear would happen .you only want to use new product.s only on flat tappet ,you can get away with replacing only lifters on a roller cam .
roller cams are made out of billet steel generally .
flat tappet and hydraulic cams are typically are made out of nodular cast iron .

 

Also, rollers are quite a bit heavier, one should consider springs for the additional weight and higher revs. Customarily, rollers are used to reduce wear on a very high lift Cam.