>You didn't provide enough information to answer you question. Is PK clustered? Are there other indexes?

Primary keys are clustered. There are other indexes.

Documentation says that the drop of a primary key will create an overhead as SQL Server has to reorg its way of finding records. I ended up doing this:

ALTER INDEX [ClientNumero] ON [Client] REBUILD WITH (ONLINE=OFF)

This allows to do it direclty without a drop, with an online clause to be off to provide more performance.