I have a SUN EZ3 AX trike (2006 model) and I took it out for the first time several days ago. I enjoyed the ride and am sure it will get easier as I get conditioned but it was difficult to get up "slight" hills. Perhaps I don't know the right gears to use. And, my knees are feeling a little tight today (5 days after riding). Again, perhaps it is just conditioning. I am in pretty good shape (I run, exercise, etc.) but we all know that bicycling uses different muscles. Also, I was thinking of changing out the pedals to the type you slip you shoe into for more efficient pedaling. Can anyone comment on changing out the pedals and also the gearing to use for hill climbing.

Changing to Power Grips or clipless pedals will be very beneficial on a trike. It's as if they were made for each other!

Gearing isn't so important, as long as you keep your cadence (pedal rpm) up. Don't try to mash the pedals to get up the hills, shift down and "spin" your way up. Most people will recommend a pedalling cadence of around 90rpm.

Tread Along, make sure your X-Seam is adjusted while in your bike shoes for starters. It sounds like you are pushing the pedals like on a DF, on bents you are much better off to "spin" as Jeff-O said. I have found that by working through the gear range my best spin is when I can just fell a slight pressure on the forward part of the spin as I am pushing through the cycle and pulling on the other foot. I prefer SPDs or "frogs" once I get the cash for them. It's also a lot safer you wouldn't want a foot slipping and going under your seat on a downhill especially.
I ride a Burley Koosah but plan to sell it (any takers email me for a great price-jman7659@grandecom.net) I took a long test ride this morning on a Sun EZ Tadpole CX that has not sold at the lbs (yea!) They gave me a sale price $400 lower than what I have seen listed in most shops, I guess they are tired of draging it outside every day <they cater to the Trek crowd mostly>The area I test rode in was full of speed bumps and drainage dips and some parts had loose gravel. After a few min. I was finding the shifting came very naturaly on the tadpole and the bumpy roads were no match for the Kenda Kwest tires, also the trike would straighten itself out almost automaticaly!. The trike was built very solid and when I gave it a test loading up the little ramp on my Toyota Truck, I was sold. My back and legs have been at their worst this week <I have a neuro-muscular problem>which is why I wanted to see if I could manage the trike under conditions that I couldn't deal with on the Burley. It was a peice of cake as they say.
I only said all this to let you know, just give yourself time and make sure the trike is set up with spds,straps or frogs, you want positive lock in on the pedals. Get used to the feal of a good spin with just enough pressure to move the pedals. Gain your speed or climbing by increasing the spin with good gear managment and not brute force. You will find your new tadpole a blast to go out and ride what ever type of roads your in the mood for.