How do we convert our drop-in sink to an undermount one?
Matt, there is a huge difference between an under mount and a drop in fixture. The first concern is the counter itself. what kind are you using?  Some counter tops cannot be used with an under mount sink.  The support and sealing of an under mount sink is completely different than a drop in.  The holes for the faucet need to be in the counter, not the sink unit, and the edges of the sink unit have to be flat and are usually epoxied and clamped in place while the counter is upside down. If you are very clever, you probably can fit the drop in under the counter, but how are you going to finish the counter edges and drill your fixture holes? They will need to be polished. This is usually a production job, not a DIY unless you give the counter maker an exact template, not something you can simply cut out and do yourself.