Any given element can come in different sizes (Timaeus 57c-d), since there are many ways to construct its basic side besides the "primary" way given above.  For instance, the primary fire particle has 24 atomic scalene triangles.  A smaller version can be constructed from 8; a larger version from 32.

Transformations between the elements can be explained by breaking a given element up into its constituent parts and recombining the parts (Timaeus 56c-57c).  Thus one water particle (20 sides) can be broken up into a fire particle (4 sides) and two air particles (8 + 8 = 16 sides); or two and a half air particles (8 + 8 + 4 = 20 sides); and an air particle (8 sides) can transform into two fire particles (4 + 4 = 8 sides).  Earth particles cannot be transformed into the others, since earth is made up of isosceles triangles (Timaeus 54c, 56d).