NOTE: I made this on a game that includes the altered sprites from the Miranda Class (In the thread called 'Help with Importing ships'), I am not sure if it works without those altered sprite files.

//*********************************************************************************************************
Open 'all_cap_veteran' (C:\Program Files\Activision\Star Trek Armada II Fleet Operations 3.0\Data\odf\system) and add at the end:

fleetcapclassXXX = "fed_nebula_carrierr6"
fleetcapclassXXXcapvalue = 6
fleetcapclassXXX = "fed_nebula_sensorr6"
fleetcapclassXXXcapvalue = 6
fleetcapclassXXX = "fed_nebula_warp1r6"
fleetcapclassXXXcapvalue = 6
fleetcapclassXXX = "fed_nebula_warp2r6"
fleetcapclassXXXcapvalue = 6

Replacing XXX with the number 1 higher than the ship before it.

//*********************************************************************************************************
Open 'all_veteran6' (C:\Program Files\Activision\Star Trek Armada II Fleet Operations 3.0\Data\odf\system) and find:

wpnname = "Promote to Veteran"

add above it

replacementXXcondition = "fed_nebula_carrierr5"
replacementXXclass0 = "fed_nebula_carrierr6"
replacementXXcondition = "fed_nebula_sensorr5"
replacementXXclass0 = "fed_nebula_sensorr6"
replacementXXcondition = "fed_nebula_warp1r5"
replacementXXclass0 = "fed_nebula_warp1r6"
replacementXXcondition = "fed_nebula_warp2r5"
replacementXXclass0 = "fed_nebula_warp2r6"

Replacing XX with the number 1 higher than the ship before it.