We're moving my daughter from a preschool she has been comfortable in to a Spanish immersion program. She didn't love her preschool, but she was comfortable there. As she was intensely shy for about a year and a half, I agonized over this decision. She seems to have outgrown the shyness and I don't want to do anything to send her back into it. We involved her in the decision (as much as you can involve a 4 year old). She felt that it was her decision to go to the new school and she is excited about it. I'm still expecting a meltdown after the first day or two when she comes home, announces she doesn't understand a thing the teacher says, and announces she is not going back. For now, she is excited about all of the new things that she will get to do. She also feels like it is part of "the big kids' school" rather than an extension of day care.

It sounds like your child is going into 4-yr-preschool this fall. Would your child be attending the new school for Kindergarten, or the existing one? If he/she is going to be going to the new school, I'd say make the change now. Young children are quite resilient, and if you present it as "older sibling will be in the same building as you" then I suspect it would quell any fears that the child might have about moving to a different building.
You might also like to join the Preschool Parents Group at community.greatschools.net/groups/11534