Convair 340/440s that were converted into 580s with Allison 501 turboprops are still officially designated in their type certificates as 340s or 440s, and the 580 conversion is covered by a Supplemental Type Certificate. The "580" is only an unofficial marketing designation. The same applies for the R-R Dart conversion of 240s (into 600s) and 340/440s (into 640s).
The STC covering the 580 conversion contains the following note.
Note II. Airplanes modified in accordance with this Supplemental Type Certificate are frequently referred to unofficially as "580's" or "580A's"; however, for official record purposes, these airplanes retain their original Model 340 or 440 identity.
However, DC-8-61/62/63s that were modified with CFM56 engines officially became DC-8-71/72/73s and are so identified in the main DC-8 Type Certificate Data Sheet along with all the earlier DC-8 models.
I'm just curious what would acount for the different handling of the Convair and DC-8 conversions.