The President can't actually change military policy with a tweet:
This morning there is news that there will – for now – be no change in the US military’s policy toward transgender service members. The news comes in the form of a letter shared with members of the press from Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford.
On its face this is no more than a statement of military command protocol and the chain of command. The President is Commander-in-Chief of the Armed Forces, with vast powers that position grants him. But the President does not and cannot just dial up the head of Central Command and order a war. There is a specific and statutory chain of command. Under the 1986 Goldwater-Nichols Act....
There’s process and there’s law and there’s the President. The person of the president does not trump the other two.
So for now, the military can remain open to anyone who is fit to serve. Let's see if the president actually orders the change through channels.