Quick copy (instead of move) for permissions

I am:

• user: userA


• groups: ... users ...

I see:

.
└── [drwxrwsr-x userB   users   ]  testdir
    ├── [-rw-rw-r-- userB   users   ]  testfile1

I can run cp testfile1 testfile2; rm testfile1 to get:

.
└── [drwxrwsr-x userB   users   ]  testdir
    └── [-rw-r--r-- userA   users   ]  testfile2

So if I have the following abilities

• read testfile1


• delete testfile1


• create testfile2 with different permissions

and because cp must read & recreate the file (I/O intensive), why can't I just mv testfile1 testfile2 without preserving permissions? Or is it possible that I don't know how?

For backstory read Can I allow users to chmod a file not owned by them?