Judging from the NCC number given for the only Wambundu class starship listed in the ST Encyclopedia (USS Drake, NCC-20381), this ship would have come before the Ambassador class (most listed with 26000 series NCC numbers).
Since the primary hull looks rather Excelsior-ish, perhaps the first Wambundu was a modified Excelsior, a test-bed vehicle for the new Ambassador-style nacelles. Perhaps this test-bed design was so successful in and of itself, that a number of Excelsiors were converted to the Wambundu design.
Just my $.02