Question: How hard would it be to re-enter a military service, or join a different one?
Answer: Prior-Service enlistments are not as easy as one might think. Each of the services limit the number of prior-service enlistments (this includes those in the Guard and Reserves who wish to enlist on active duty) it allows each year.
This is because a "prior service" enlistment slot is the same as an "re-enlistment" slot. So, in order for a prior-service to enlist, the service must be under their goal for re-enlistments.
For more information, see the article, Prior-Service Enlistments, and Transfering from the Guard/Reserves to Active Duty.
