All sites reset when changing the site version. Therefore, it is recommended that you change the default value to the one you add most.
The default value will not affect existing sites, .net, classic or otherwise.
The only problems you may encounter is that you cannot run the virtual application 1.1 on the 2.0 site.
In addition, you cannot run different versions in the same application pool. Therefore, you must change the default application pool in the same way as changing the default version.
As an aside, I read some command line parameters and other settings, which mean that you can change the version of the site without calling all iis to reset. However, I did not succeed personally.
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