It's true, if you change all of the properties of a guard tower to make a base tower, it saves you a ton of work in the long run.
Here's what you do:
Just take the Guard Tower, and modify all of the properties that you will need to change later (food, lumber, attack, model, sound, etc) and change them to something random. What's the point? So that they show up pink, so that when you copy and paste the tower to begin working on your towers, you can easily find what you need to change.
Since i'm developing a TD now (Well, on and off for a long time) I've found that this is probably the fastest way to do the incredibly slow and boring process of object data.
Good luck!