DB Forge: possibility to change / set a primary key on a field of an existing table?

Hi all,

I was reading the user guide on the CI4 Database Forge class.
I want to set a primary key on an field of an (existing) table. 

The addPrimaryKey() should be use on fields that must be created, but is there a possibility to do so on an existing one (by mesans of modifyColumn() for example...)? 

Thanks in advance!