When stem cells transforms to form cells characteristic of other tissues, the process is called as -
Transdifferentiation takes place when a non-stem cell transform into a different type of cell, or when an already differentiated stem cell creates cells outside its already estabilished differentiation.
Remember very important fact that it is the non-stem cell or already differentiated stem cell (i.e. mature cell) that is transformed into other type of cell. It is not stem cell that is transforming.
Transdifferentiation is a type of metaplasia.
Then, what is the difference between transdifferentiation and metaplasia.
In Transdifferentiation only differentiated stem cell is transformed into other cell type, while in metaplasia any of the two, either stem cell or differentiated cell can transform into other cell type.
So, all transdifferentiation processes are metaplasia, but not all metaplasia are transdifferentiation.