You really picked a great season to model. When I was in college, my HO railroad was set in the fall of the year. I really loved the fall maple trees!
You may be able to keep your grass mat as an under base for your new scenery effects. Look in the automotive department of your local big box store. There should be large cans of spray paint there. Look for the flat camo colors people buy to camo their pickups and Jeeps. There should be several shades of tan, supposed to look like, well, dried grass.
Lightly dust this spray over your grass mat (I am thinking it is made of dried sawdust, like the kind Life-Like sells). Try not to get an even effect. If you have more than one shade of tan, so much the better.
Weeds and other tall growth can be purchased from the hobby shop, but you can get the same stuff cheaper if you go to a store that sells materials for craft projects. Try Hobby Lobby or Ben Franklin if you have them. These are a good source of items that you can "imagineer". For instance, I've used small glass beads for insulators on phone poles or around a power plant.
The ready-made fall trees are lovely, but costly. Be sure to get some at least to use in the foreground.
Lichen has been pretty well replaced by ground foam and polyester, but it still looks good as background cover. If you can get a bunch of it dyed in fall colors, you can build up a background hill out of chicken wire or styrofoam and glue clumps of the lichen to it. Again, if you buy the lichen in the craft store, it should be cheaper. You might even try spraying the clumps with fall colors before attaching them.
I think the main problem with grass mats is that they are too uniform in appearance. I still use grass mats here and there, but only to represent a manicured lawn. If you have a large grass mat area, you might try placing small objects beneath it to create small hills. And be sure to put more foliage on top of it for bushes and weeds.
Most of the ground cover on my railroad is various shades of ground foam. You can sift it directly onto wet paint. I now model the summer in a western urban setting. Light olive acrylic makes a good underbase for late summer grass, and would probably work for what you have in mind.
Woodland Scenics makes a good line of tree materials that can save you a lot of money if you want a lot of trees. They sell a bag of 114 tree "armatures". These are brown, but can and should be painted to look more like trees. Painted gray and used "as is" they can represent dead trees (Paint an upper limb and a band down the trunk flat black for a "tree killed by lightening").
You can paint the trunk to represent whatever species you want to model (you have to go outside and look at some trees- very few are really brown). Woodland Scenics and other brands sell polyfoam and lichen foliage materials, including some in fall colors.
Go for it!
Les