I have 240k on what seems to be original transmission... I now have a built 6.0L hooked to that old Trans. Is running like a champ. I am doing a built transmission.. However I wouldn't bother replacing for preventative maintenance.
Here's what I would do...
Get a Trans temp Guage or swap cluster for one that has Trans temp.
I have done some pretty intense stuff, and my temp has never gotten much over 190.. typically is much lower. If you seem to be towing, climbing off road, etc and getting higher Temps, then install a larger Trans cooler. Will extend life of transmission.
Closely monitor fluid... Pull dip stick and while on white towel... should be pink color. If otherwise change fluid and filter. Some ppl say may wreck old Trans but I haven't ever had that problem... but if 8t does probably needed new / rebuilt transmission anyways.
People bash of 4L60e... I am tearing one down that blew up... It looks checkball messed up valve body and starved of fluid. on top of that was Frankenstein rebuild by somebody who didn't care about quality. probably what killed it to begin with.
I think the quality claims for 4l60e are mostly baseless people who didn't maintain them. I have had half dozen vehicles with them, beat the snott out of every one and never had major issues. Think worst was solenoids in my z28 camaro. They are a great transmission if anything clutches maybe a bit weak but other than that design is solid and proven over decades of production.
If you ever get a rebuild make sure is by a reputable shop, none of those Craigslist specials. I think they throw junk parts in the Craigslist specials. Should be about 500$ in clurches seals etc plus whatever else was needed for rebuild... I would expect good builder to be around 2 grand for a 4l60e... but not under 1000$
Hope that helps.