Ceramic tile with particle board underlayment

I am planning to install new ceramic tile in a bedroom converted to a bathroom (12 ft by 10ft). The 20-year old house is a pier and beam with 1/2" plywood subfloor, 1/2" particle board underlayment and then 1/2" plywood underlayment on top of the particle board. To keep the floor height level with the hall, I was going to pull up the top 1/2" plywood, then put down 1/2" plywood and then Hardibacker...and finally tile.

Reading a previous post, it sounds like I should also pull up the particle board and then put down 3/4" plywood...then the Hardibacker and tile. While I know I can pull up the top 1/2" plywood...I'm not sure whether the particle board may be glued to the plywood....thus adding alot of extra work to remove it.

Before I dive in and tear out both layers...can I put a moisture barrier between the 1/4" plywood and particle board to reduce risk of water damage? Saw RedGuard mentioned in a previous post...not sure if it would be applicable here?

Thanks in advance!