I have found a way to do this:
You need to open TS2009, or whatever your older version of TS is.
Create a clone of the asset that is being replaced in TS2010.
Give this cloned asset a new name, that you will be able to find when you look for it in TS2010.
Once you have all of your cloned assets in the previous Trainz, export them as .cdp (you can put them all into one file).
Open TS2010 CM3, and import the .cdp you created in the previous version.
You may need to fix the imported assets. Some will have missing textures.
Use PevSoft utility Texture2TGA to fix this.
Open your route in TS2010, and use the Replace Assets feature from the main menu to replace the Speedtrees, or whatever assets were changed to the ones you want.
Note: Back in CM3, there are some cases where you might get errors or warnings about .pm files not being supported in TS2010. If they are just warnings, first try ignoring them. But if the assets don't open, then you need to convert the .pm to .im, using another PevSoft utility; pm2im.
If you need help on installing and using the PevSoft utils, there is a tutorial available somewhere on this forum. It is a bit tricky for the every-day gamer, but it works very well, and is easy to use once it is set up properly in TS.
I can help you with this, if you need, but it might take me a little time to get to it.
Good luck.
FW