Since the F5 and the ACE can't share information, if you bring the VIPs online in the F5 before you disable them in the ACE, you will have a service interruption and IP address conflicts...
When we migrated from ACE30s to F5s, we used two methods depending on the tolerance of the particular user base.
- DNS. Using new network for the VIPs on the F5, we cut the vips by changing the DNS answers.
- Network. Using the same addresses we simply removed the vlans from the ACE, cleared the MAC and ARP caches, then trunked the vlans into the F5.
There are issues with both.
With 1) you have a period of time depending on your DNS ttls, where traffic will be split between the F5 and the ACE. You also have issues as there are some systems that will not honor the ttls etc.
With 2) you will interrupt service. No question about it. It your customer has redundant Data Centers, this isn't a big deal as you can bleed traffic away. If not and they can't tolerate any impact, you're pretty much stuck using alternate address space.
I'm sure there are other options, but our scope didn't leave much room for further creativity.