You can cast a Keras variable but it still returns a Keras tensor.
k_cast(x, dtype)Keras tensor with dtype dtype.
Keras tensor (or variable).
String, either ('float16', 'float32', or 'float64').
This function is part of a set of Keras backend functions that enable lower level access to the core operations of the backend tensor engine (e.g. TensorFlow, CNTK, Theano, etc.).
You can see a list of all available backend functions here: https://keras.rstudio.com/articles/backend.html#backend-functions.