As we saw when we looked at the benefits of keyed alike, Master and Grandmaster suites, their availability rest on a skilled locksmith's ability to pin a padlock for a specific key or set of keys. The provision of semi-assembled and re-keyable padlocks is what makes this possible:
Re-keyable padlocks differ from semi-assembled padlocks in that they come in full working order, but the cylinder mechanism can be removed and the pins replaced. An example of this is the Kasp™ 190 series lock which can be re-keyed in either keyed alike or Master key format.
This ability to re-key doesn't affect the security of the lock in any way as the cylinder can only be accessed for re-keying once the padlock is already open.
Sometimes called a service lock, this is a padlock that comes assembled except that it has no pins in the pin tumbler cylinder and the keys are uncut. Locksmiths use these locks to achieve a specific bitting, such as when making keyed alike padlocks, or to make Master/Grandmaster suites.
The way they do this is by inserting the various sized pins and springs into the holes in the side of the lock body. Once all the pins and springs are inserted the holes are plugged and the side of the padlock is refinished to the standard of the rest of the padlock.