Hey Larry,
You can pseudo-transfer. You have to login to Sunpass.com and disable the old one and request a refund to the credit card (it actually doesn't have to be the same credit card you used to fund it, believe it or not).
In case you didn't read my latest regarding the Sunpass Slim, it doesn't actually work below the mirror. I was lucky that it worked two times in a row, and it has been about 50/50 since then, and it doesn't work at certain tolls ever. I *can* get 100% by holding exactly as follows (this can't be mounted, and you have to hold it with one hand):
1) The Slim is held horizontally
2) The left end of the Slim is against the right side of the mirror mount (the much shorter side), as far as it can go without passing the mirror mount post
3) The right end of the slim is against the window to the right of the mirror mount.
It's a freaking pain in the ass...