Yes, they say it's PRIME UNIVERSE, but that means they're retconned and rewritten an lot. Let's take a look ahead at Disco's version of TOS and speculate about what TOS might look like today... We've already seen that the USS Enterprise herself looks different with movie-style swept-back nacelles. We've already seen the Pike-era uniforms apply the Star Trek Beyond colours and rank stripes, and black collars to the Disco costumes. The arrowhead is a badge now, and the miniskirts have black tights underneath. The characters obviously now look like their modern versions, with Anson Mount as Pike, Ethan Peck as Spock etc. Klingons likely still have bumpy heads and purple skin, and although it's likely ones looking more human will appear too they will be one type among many rather than the norm. "Balance of Terror" would occur differently since cloaking devices are well known in this era, and the Romulan ship would certainly have warp drive also. Would the Romulan Commander look like James Frain? Instant communication across the cosmos will also alter "Balance of Terror" in that Kirk won't make the judgement call by himself to enter the zone... or they'd set the whole thing in the green communications-jamming nebulae on the Romulan border from Nemesis. "Errand of Mercy" wouldn't so much be a cold war turning hot as it would be resuming open hostilities, with comments to that effect. Perhaps the Klingons shave their heads again briefly. Holographic communication will alter episodes where Kirk and company spoke on a little monitor to the Admiral of the Week, he will be in the actual room speaking to them. "Yesteryear" would either have Micheal at the Sarek home or some excuse why her (and Sybok, for that matter) aren't. "The Slaver Weapon" would be extensively different, to fit with ENT's version of Trek prehistory. What else?