Even watch shops will inevitably leave scratches. The people at Rolex do many watches everyday, and they still leave scratches. Don't think just because they do it often means that they will do it perfectly. If you have ever done it yourself, you would understand that it is a very delicate process.
If you're taking off a metal bracelet, especially for the first time, I'd say there is a 99.99999% chance that there will be scratches. Changing out a regular leather strap on regular lugs (on a Lange for example) would be a lot easier and chances are much higher that the lugs will remain unscathed. Even then, just a small slip will leave a scratch.
If your watch comes with straps that have tool-less spring bars, then yes, the chances of scratchless strap changes are very high. But you'd also need to buy another strap with tool-less spring bars, which is extremely limiting in terms of choice.
I say if you really want to change straps for your watch, adopt a "it's just a watch and a few scratches where nobody sees doesn't matter" attitude. Otherwise, don't touch it.
