Cluin- Hey, I'm a Guild fan. I'd recommend an off-board pre-amp instead of the hassle you have with changing batteries. It would require a bit of wiring rework, but I bet you would end up with a better sound. Bagg's PADI is a great pre-amp, with features you probably don't have with your on-board setup. Off-board pre-amps run the gamut from expensive rack-mount units, to little boxes you plug directly into your guitar.
You also might be able to snip the battery leads and bring the battery out closer to the soundhole, to facilitate battery changes. You'd have to be able to remove the pre-amp, so you could solder the wires.
If, by some chance, your battery plugs directly to the pre-amp circuit, with no wires involved, your solution is even easier. Go to Radio Shack and buy a couple of 9 volt battery terminals. Measure the distance from the pre-amp to the edge of your soundhole, then cut and solder the two terminals together, respecting polarity, of course. Clip one end to the pre-amp, and the other end to the battery, which you will mount just inside the soundhole with double-stick foam tape.
In any case, keep the Guild. IMO, any of the above solutions, or even staying just as you are now, are probably better than buying most of the junk out there now, unless you're willing to pay big bucks. Any questions, PM me. thanks, BlueJay