Be careful with that Decathalon. Trainers almost always have the CG marked really far forwards, and they have big tail surfaces. Decathalons aren't trainers, and are usually more sensitive to CG. Some Decatholons that I've seen have been very close to tail heavy even on the suggested CG range (One, I think it was a GP kit, was down right "snappy"). But if the previous owner had it on the suggested location, and the plane flew like it was nose heavy, then yes, you should try moving it back. Just do it in small increments.

Just to clarify, I make a first flight with the CG where the plans say unless I have a reason to do otherwise. It's just that you can feel free to adjust it from there.

Once the plane has been flown, I ignore the instructions and make small adjustments until the plane flys like I want it too, or is close enough. Think of the plans as a good, safe, place to start, rather than the absolute final CG location.