Want to know how to play any single player lvl with just the trial? Read on

First, make backups of spacestation map and which ever map u want to replace it with.
Always start this from the original files and not renamed ones if you have already done this before.

u will need a resource hacker/editor for this. Use google. I used ResHacker.

Ok open up startup.exe in your resource hacker. Go to the section that is shown in this picture -> http://img259.imageshack.us/my.php?image=capturerz2.jpg

Now, what u have to do is scroll to the bottom of the right side untill u see the single player map files. 01b_spacestation.map is the level you get to play. Lets say i want to play new mombosa (3nd lvl in the game)
I look at the sizes of both the spacestation map and new mombosa map and swap them in res hacker. (so if the size for spacestation was 100 and new mombosa was 500 then spacestation will now have 500 and new mombasa will have 100)

Now you have to save the .exe, Vista will probably stop you from overwriting the original startup.exe so just make it startup2.exe if you have to.

Now navigate to your maps folder and rename 01b_spacestation.map to 03b_newmombasa.map and
rename 03b_newmombasa.map to 01b_spacestation.map.
http://img259.imageshack.us/my.php?image=capture2df2.jpg

Finally run startup2.exe, click activate later. Go to campaing, and choose "Select Level", Select "Cairo Station" http://img259.imageshack.us/my.php?image=capture3fv8.jpg , and enjoy http://img259.imageshack.us/my.php?image=untitled1hi4.jpg

Note: You can do thi with any single player lvl (haven't tested MP ones yet), just follow the same instructions as before but switch new mombasa with which ever map you want.

PS: Don't wonder why my sizes are different than yours, mine was already swapped when i took SS.