I got new wall mount AC Adapter, the adapter itself is good, but cord is low quality, very short (1m) and very hard and coarse, non-flexible. I want to replace adapter cord with better one. The adapter have ABS case without screws, two halves possibly are glued or thermo-welded together. Does anyone know whether it's possible safely and accurately open such plastic cover with knife and replace cable? I just don't want to damage the item in any way. I can try to return adapter to but its looks difficult.
I would not open the adapter at all. Cut the cord a few inches from the adapter, and splice on the cord that you want. You can do the same thing at the other end if you want to continue using the existing plug.
This has the advantage that you aren't messing with, and possibly defeating, any of the safety systems. All the work you are doing is on the low voltage and on the safe side of the isolation barrier.
Safely? Yes. As long as it's not powered and fully discharged. Just take extra precautions to check connections before resealing and use a good adhesive at the seam.
Accurately? Probably not. There's a thin chance you'll get lucky and it will have a snap together design that can be popped open with some slotted screwdriver pressure. Far more likely that it will be glued shut.
I have opened similar, but almost always at the expense of some digging into the plastic to get it separated and having to glue it back together after. Some light heat gun application can sometimes loosen the adhesive, but rarely enough for an easy clean break.
Considering the odds of it getting a little ugly in the process, and the issues with strain relief (as mentioned in the comments), you're probably best off exchanging it if you still can or making a more flexible extension cord.