In reply to Graeme G:
I checked out your bike on the Trek Archive site
https://archive.trekbikes.com/us/en/2006/trek/8500#/us/en/2006/trek/8500/de...
Your going to have spend ££££s to get something as light as your current bike, unless you've found yourself struggling on steep downhill sections I would keep the old bike because that is the only terrain that bikes with modern geometry make easier to ride.
If your fork bushings aren't worn out and the stanchions aren't damaged, your Fox fork will be good as anything they make now once its been serviced. Your only options for replacing it would be secondhand anyway as no-one makes decent forks with straight 1 1/8" steerers anymore. I borrowed a mates new Orange 5 last year with Rockshox forks and they weren't any plusher than my 2002 Marzocchi Z1s.
I would fit a brand new 11 speed XT 11-46 cassette and a 30T or 30T single front chainset, which give as low a bottom gear as the 22:34 bottom combination on your current 9spd triple. Modern single rings matched with one of Shimanos clutched derraileurs means dropped chains are a thing of the past.
Brakes haven't got any better so if the old ones still work keep them.
26" wheels are marginally slower rolling than 29", but unless you're racing you aren't going to notice.
Tubeless tyres and wheels are the best upgrade I ever made, large tyres 2.2"-2.5" with low profile tread roll much faster with more grip than 1.8"-2.2" nobblies, unless you live somewhere with really claggy mud where you need clearance.
If you've not already got a carbon fibre handlebar, I'd recommend getting one they damp out a lot of the buzz and reduce fatigue.