Sounds like you're at the wrong shop.. I guess if its Trek its probably some big store with lots of low knowledge staff. Go there when you know what you want and looking to save money.
If you want good service and knowledgeable staff, go to a small shop and you might at least get someone who can talk you through it, that's usually what I've found. Might cost a bit more, but they should be able to hook you up with exactly what you want. I find these type of stores are willing to help me find parts for any old piece of crap bike, as they just have a passion for this kind of thing; hence owning/toiling in a bike shop all day!
Any store is going to have ordering policies, based on frequency (once a week?) or when they have a large enough order to place with the supplier. Keep that in mind.
So you have a Trek hybrid bike.
It has an 8 speed groupset with 700c wheels and disc brakes?
To me, that should absolutely fine on paved roads, etc.
Your problem is you want to ride on trails?
As I see it, you have a couple options:
1. Buy a 26" disc wheelset with + 12-25 Shimano/SRAM cassette and swap with the 12-32 8 speed cassette + tires. ~$250+ (all parts + labour or tools)
Tools would include a
shimano cassette release tool and chain whip (click link for more on that).
Advantage: Low cost, easy to swap out the wheels. Your road wheelset has 12-25 rear and your MTB set has 12-32 rear. Learning about components/perhaps about how to upgrade/ repair.
Cons: Not as simple/easy as option 2. Hybrid geometry is not the best suited to trail riding. A bit of wrenching and tools involved.
MTB tires might not even fit your frame. Measure the width to see if there is enough clearance, and compare with the width of the MTB tires you want.
2. Buy a mountain bike. ~$300+ (used, decent hardtail bike with cable disc and front suspension fork)
Advantage: Easiest option. Made to ride on trails. No need to change wheels, or adjust drivetrain before riding. No wrenching.
Cons: Could cost a bit more.
Other than that, I'd say upgrading to 9/10 speed or any other upgrades are not worth it and border on the territory of simply purchasing a complete bike.
Before you go to the store, look at classified ads for used 26" disc wheelsets. Make sure they have disc brake mount, and a shimano/SRAM cassette freehub. Bonus if they come with an 8 speed cassette. Also see if there are any affordable used mtb's.