Well, ideally you'd do best to upgrade the front 3 speakers. However, I would suggest if you want Axioms to go with either a pair of M3Ti speakers or M22Tis.
I guess another option would be to go the BIC route. The Venturi Center is very highly rated...and inexpensive! You could replace all 6 of your speakers for under $500 if you went with the Venturi line.
This system would be:
2 DV-62SIB bookshelves for the fronts
2 DV-62SIB bookshelves for the rears
1 DV-62CLR for the center
1 DV-62CLR for the rear center
You can get these all on Amazon with free shipping for $451.86.