Gue'vesa tanks may have few design changes.
It would need refit of industrial tools to integrate heavy Tau tech (anti-grav, energy powercell and weapons) on Imperial vehicles. So unlikely to see a Railgun or anti-grav plates on Leman russ.
Long time gue'vesa may use Imperial vehicles, but maybe with Tau secondary tech (comms, sensors, missiles, maybe small arms up to battlesuit class weapons). They may even be trained to make and use Tau vehicles. Or the new Tau tech mind could see imperial engeeners doing something outside of copying millenia-old STC and create new vehicles with the help of Earth caste and get rid of visible bolts, aquilas, WWI-style tanks, ...
I feel right for gue'vesa to use some middle-tech sci-fi style vehicles, weapons and armors (Mantic Enforcers
, Puppetswar Federation
, Anvil Industry human armies
Our fellow El'mo done some guevesa tanks based on puppetswar tanks that feel less Imperium and more "Tau"/modern sci-fi http://www.advancedtautactica.com/viewtopic.php?f=42&t=19447&start=50#p316583