On my last TT, I didn't need to replace plywood, I did need to add some. The 200 lb weight rating, was not enough for my plan to use the TT at deer camp. I have two boys who are over six feet and 250 lbs.
I added a layer of 1/8" plywood and installed some moldings for additional support under the upper bunks.
After the mod, the boys said the the bunks were FKA (fat kid approved).
For replacement, I would replace that 1/8" playwood, with 1/4" plywood. You'll only add a couple pounds and the bunk will be much stronger. If it is an upper bunk, consider adding some vertical support.