Isomerization
For example, when a ketose group changes to an aldose group, that is isomerization because both groups are -C2H3O2.  They have two carbon atoms, three hydrogen atoms, and two oxygen atoms.  In the ketose group, the first carbon has a double bond to an oxygen and a single bond to the other carbon.  In the aldose group, the second carbon has the double bond to the oxygen, and the hydrogen atoms are in different places.  But they both have the same number of each kind of atom.