Renault scenic 1999 automatic gearbox revs at 5-6k before it will change into second gear, and that is if l drive off slowly. Sometimes if l lift off the accelerator a little it will change at 4500rpms.

l have a feeling the gearbox may be sealed for life, but i'm wondering if the problem could be something else, a sensor maybe?

After a little guidance before l go to the renault dealership.

put some ATF in it before it is totally burnt out.

There is absolutely nothing u can do yourself with an auto gearbox, except maybe check / top-up the fluid level, but even that is a pain on some cars. It definitely sounds sick, since it should change up at about 2000rpm on a light throttle, but we'd be wasting our time here trying to think of a possible cause for it.
An automatic transmission is by far the most complex device fitted to any car, & it is repair should either be taken care of by an authorised dealer, or if it is out of warranty u could try looking up Yellow Pages for an automatic gearbox repairer in ur area.

Renault autos from this era have a bit of a reputation. You could take it to a dealer of preferably to an auto specialist. Check your bank balance first though as it will be expensive wherever you go

Its bad news dude.
Renault Automatics = nightmare.
Spent so much on having my Clio automatic repaired l ended up having to sell it just to get my money back on what I'd spent out on it. And Renault are the only people who can tell you whats wrong and then charge you a LOT for the priveldge.
Sell this car before it goes seriously wrong, and save yourself a LOT of trouble.