Change Application Pool on SharePoint IIS site

To change the application pool for an existing Windows SharePoint
Services site (IIS virtual server):
-Ensure that the new application pool is running under the same
identity the old AppPool is running as. (if you don't, then SharePoint
may not be able to connect to the content or configuration database)
-In IIS, expand Web Sites.
-Right-click on the virtual server you want to change and select properties.
-On the ‘Home Directory' tab, under the ‘Application settings'
section, change the ‘Application pool:' to the new application pool
and click Apply, then OK.
-Back in the Internet Information Services Manager, expand the virtual
server you are changing by clicking the + sign beside the name.
-For every Virtual Directory (default virtual directories for a WSS
site are _layouts, _vti_bin, and _wpresources)under the website,
ensure they are running under the new application pool by:
–>Right-clicking on the virtual directory, select Properties.
–>Under the ‘Application settings' section, change the ‘Application
pool:' to the new application pool if necessary, and click Apply, then
OK.
-Reset IIS