You can't actually rewrite the url portion of the uri, but in your example you are actually looking to rewrite the host header which the ace can do. Here is how you do it:
1. make a parameter map to tell the ace to look at each request for rewrite eligibility:
parameter-map type http rewrite
header modify per-request
2. Make an action list to do the modification
action-list type modify http host_rewrite
header rewrite request Host header-value "www.cisco.com" replace "www.yahoo.com"
3. apply the action list to the load balance policy:
policy-map type loadbalance first-match test4
class class-default
serverfarm test4
action host_rewrite
4. call the load balance policy and parameter map in your L4 policy map for the vip class.
policy-map multi-match test5
class test1
loadbalance vip inservice
loadbalance policy test4
loadbalance vip icmp-reply active
appl-parameter http advanced-options rewrite