The slide metal contracts, makes the dovetail slot bigger.
This is not correct. The slide gets smaller in all dimensions. If we measured from front to rear, the slide gets shorter. Each segment along the length of the slide also gets shorter. This includes the ejection port, firing pin slot, and the dovetails.
Now it may be true that the sights shrink even more than the slide, and thus are easier to remove than from a hot slide.
I am mostly interested in being able to change the front sight(I really like my Ruger gp-100 for that - I change it quickly and easily). I see that the in sights are dovetailed; does that mean tapping l-r every time one is changed, or does the front at least have one spot it goes in to?
There is no dimple or anything to precisely set the position of the sights. They can be moved with a sight pusher or they can be moved with a punch and hammer. The aftermarket TruGlo TFO sights were easy to move with the Sig sight pusher, but the rear factory sight is too wide for the Sig tool, so I had to knock it out with a punch. If you are a bit handy with tools this is a pretty simple job. There have been some other threads on this subject with some good details.
I don't think it would be wise to repeatedly change the front sights, because the sight bases are softer than the slide and they get a bit distorted when installed. Repeatedly installing and removing the same front sight will quickly cause it to be loose.