We had a pair we used when my 12u would play up. Nike's. They came with one set of each. I replaced the molded spikes on them about 3 times during the season. Ran about $9-$11 for a replacement pack. Would put the metal spikes on when playing up. Never had to purchase replacement metal spikes.
They worked great and you're able to keep a fresh set of molded spikes throughout the season. Usually takes about 10 minutes to swap a complete set on a pair.
Currently, one of my boys has a pair of New Balance metal cleats. The pair he has are not convertible.