From what ive heard in the DCS F-14 discord server (its a very nice place for finding out rare or obscure information on the tomcat. Ive been researching it for over a decade now and i still pretty regularly find out new things there), the designation change was due to a new logistics software freaking out over the A+ designation and at random sending A+ parts to A squadrons and A parts to A+ squadrons. And it ended up that it was easier to re-designate the F-14A+s to F-14Bs than to commission a new logistics software.