There are a fair amount of in universe upgrades

The tank can also be equipped with camouflage netting, extra armour plating, a Hunter-Killer Missile Launcher, improved communications equipment, a mine sweeper, a Pintle-mounted Heavy Stubber or Storm Bolter, a searchlight, track guards, rough terrain modifications, a dozer blade, and smoke launchers. 

http://warhammer40k.wikia.com/wiki/Leman_Russ_Tank