I've never dyed fleece, but kinda think the color remover would not be necessary because
the color is through the fiber and not applied later
navy is dark enough that over blue, it should not matter - a lot depends on the type of dye you are going to use.
That being said, I still think the cost of new will be cheaper. Dick Blick carries iDye for polyester, and Dharmatrading.com may have something there.
Good article here on dying poly
http://www.pburch.net/dyeing/dispersedye.shtml