Your plan to cut the top half and cap it with the original top panel is workable.
Again, I would recommend reading up on this excellent Instructable so you can get sharp, clean cuts. Using the right blade and tools (table saw or circular saw) will greatly minimise splinters and the tearing off of the laminate layers.
We recommend these Hackers Help questions for further reading:
How to shorten PAX wardrobe frames?
I moved into a new house with a huge open closet room with no storage. I designed my entire walk-in gorgeous closet using the PAX system, but now have realized my ceiling is too low. The ceiling is 78 1/4” tall. Can the PAX system be cut down? Would you cut the bottom or the top? Can I shorten the PAX frame?
I wanted to share my IKEA hack because I thought it might give a few people with broken panels in their IKEA PAX sliding doors hope. Here’s how I cut the PAX door panels.
How to reinforce the PAX back panel?
The PAX back panels seem a bit flimsy and I am guessing that they will bulge if anything is pushed against them from the inside of the wardrobe. Does anyone have a suggestion for backing them up? Here’s the solution to the flimsy PAX backer board.
Jules Yap started IKEAHackers.net in 2006 as a personal blog to showcase the most impressive IKEA hacks from all over the world. Since then, she has learned a lot more about power tools and DIY. Her site has helped thousands modify IKEA furniture with step-by-step tutorials, craft projects and home styling.