1- The F-14B had the same F-14A avionics, except by the RWR.
2- As far I know, some F-14As were upgraded to the F-14A+(later in 1991 F-14B)standard, and some were brand new F-14Bs, not upgraded.
3- the F-14B was implemented in 1987.
If you want more details, check this link: http://www.anft.net/f-14/f14-history-f14b.htm