If I change the landing URL...

does it constitute a change in the ad?

[beren]

I have an ad group with a long history and a high historical CTR. This helps in placement and allows me to bid lower than I otherwise would. So that's good.

If I change the URL to modify some tracking code, will this be considered a change in the ad? And if so, will that mean my historical CTR will be wiped away? Because I don't want to go back to the default CTR used in calculations for new ads, as that would require me to bid higher to maintain a decent position.

[Sanenet]

I believe that it's considered to be a change in the ad (so it's reviewed again before going out to the content network) but the historical CTR still applies.

[NedProf]

I think the history is based on keyword level. So changing an ad(or destionation URL) will have no influence on the history of your keyword.