redirect traffic after decommisioning applications

if you are talking about a GTM scenario you (but same thing for ltm) you need to keep existing config. Hthe wideip, but change the wideip configuration to use a CNAME instead of responding with pool information. the CNAME configuration is in the advanced menu of pool properties menu. the behavior will be that instead of responding with an IP address corresponding to you GTM pool members, GTM will respond with a CNAME, which is somehow a DNS redirect, so here you put your new site address.