Hi igofar, first of all, I really don't think you need to replace the chain guides so often if the engine runs fine. True, distribution seems to be the weak point of the TW, but it should be okay until at least 15,000 miles. You might want to check your chain tensioner (even if it's the automatic one).
Then, you don't need to take the head off to replace the guides, but you must take down the rotor. I had the Yamaha dealer change the right chain guide only because I didn't have the tools to take down the rotor, but it's actually pretty easy and it can be done using the rear wheel axle. It's all in the repair manual! Check it out.