Looking at car-part.com for ECU's. They break the dates down not only by year, but there a 3 computers listed for both the '97 and '98 models.
(5sp listed but same happens with auto and all trim levels). '96 and '99 only offers 2 date ranges.

Searching for a '97 unit
Elec Cont Unit (ECU), thru 10/96, SE, MT
Elec Cont Unit (ECU), 11/96 only, SE, MT
Elec Cont Unit (ECU), from 12/96, SE, MT

Same thing searching for a '98 unit.
Elec Cont Unit (ECU), 6/97 only, SE, MT
Elec Cont Unit (ECU), from 7/97 thru 2/98, SE, MT
Elec Cont Unit (ECU), from 3/98, SE, MT

Can anybody speculate what the ECU changes were made DURING a model year?