Would it be hard to branch transfer from AMEDD to Cyber as a Reservist?

Sir, you'll want to reference MILPER 17-260:
Fiscal Year (FY) 18 Cyber Branch Transfer Eligibility Panel for United States Army Reserve (USAR) Officers/Warrant Officers and Army National Guard (ARNG) Officers ...Issued:[8/15/2017 3:46:32 PM].

The message indicates eligibility requirements. It is a long and detailed branch transfer packet as I've witnessed many of my officers go through it. I can link you up with many people who have completed packets, but ultimately on the Reserves side it will be how cyber you are before they let you go cyber and then depends on what the board says. I also have copies of the AARs from the officer selection boards for the panels. I've not seen or been privy to the next round of dates for the panels but know enlisted in my unit who have not transitioned need to submit packets soon.
Also worth noting as you consider this is that on the Reserves side, there are only a handful of units that you would have a slot in as a 17. Depending on where you are, I'd be able to tell you where the closest unit is.

Does Branch Transfer really happen? When I was in signal school at Ft. Gordon in 1968 I was selected and offered a spot in Signal OCS, I did not ask for this they came to me. I happily accepted. Signal OCS was also at Ft Gordon so the bus trip would be very short. Anyway, the night of the transfer there were several of us on the bus that was to be heading over to OCS. Some OCS NCO, E7 came on the bus to tell us that effective immediately Signal OCS was full and will not be accepting any more candidates. However, each of us had been accepted for an appointment to Infantry OCS at Ft Bragg (or was it Benning?) He went on to explain that once we were commissioned we would be able to apply for a branch transfer back to Signal. Keep in mind, this was fall, 1968, post Tet. I was really torn. I wanted very much to go to OCS, I really wanted to be an officer and believe I had the right stuff to do a good job. The reason I enlisted rather than wait to be drafted was to allow me the opportunity to choose an MOS other than infantry. Now, the Army, in all its wisdom has elected to send me to the infantry anyway. The choice was ours. Most of us elected to get off the bus, a few guys took the ride to the blue chord, I didn't. Now, some 50 years later, that is one of my greatest regrets. I have been second guessing myself for all these years. My life today would have been way different than it is now. I had a buddy in Korea who had a nickname "re-up Motherf...er"