I am with Clarkson and Bruno on this one. Leave the financial aspect of paying for college aside for a minute.
The amount of havoc it would place on the system would be insane.
First off, AROTC does not force you to go AD like AFROTC or NROTC, the SAs require AD and that makes it easier to manipulate. Let's say a cadet wants to go Guard with the Army, now you have to find someone not only wanting to transfer into the Army, AND in that branch, but also Guard. That is not an easy feat.
Secondly, many of these cadets will have specialty fields, and they get their career assignment for AFROTC in the spring of their jr yr. From what I understand the Army doesn't hand out to ROTC until their sr. yr. That means the AF has already predicted their manpower and they don't like messing with things of that size unless they have to.
Thirdly, the military does long term strategic manpower planning 5 yrs out. Look at AFROTC scholarships going out currently it is for graduation dates of 15...ROTC grads do not report until after SA grads, which can be 9 mos + (this now brings you to AD in 16...5 yrs from now). They know historically how many will leave the program and plan that number in now. Only when we face economic times like we have currently do we see them re-jigging, and it isn't to add onto the rolls, but to cut. For FY 10/11 they took all of that into account for Class of 15.
Fourth, IMPO, probably the biggest reason why this would be a rarity. Every branch has OTS/OTC. This is the last prong in the system, and it is created to keep the numbers in flux. Currently, the AF canceled their last OTS board because for 10 and 11 they have enough officers. They can get qualified candidates through the system in months, thus, if they needed more candidates they have this option to fall back on. Now a cadet Class of 11 who thought they could do this, would not be looking at this option currently, they would be SOL if they thought that they would accept one ROTC scholarship over another, but be able to transfer upon graduation.
Finally, look at it from a different perspective. Why should they take you over the OTS candidate? You already trying to dive on the 1st branch you promised to stay with for 4 yrs, and that was with them financially investing in your education.
Branches don't like doing this to other branches because it creates a spirit of competition and pilfering. The AF would be mad at the Army if they took their cadets, and the Army would be mad at the AF, thus, they try to keep that to a non-playing issue.
If you are unsure of the branch, it may be wise to do an in college scholarship and 1st try out the branch as a freshman to see if you want that service.
Look at choosing any branch as you would at buying a house if you have children. You buy the house because where they attend school, not with the hopes that they can cross district lines and attend a different school.