Migration of servers load balanced in BIG-IP LTM

1. Can the migrated servers land in XYZ(with new IPs), still be load balanced by F5s in Org ABC over L3 connectivity for an interim period? if yes, what changed are required to be done in F5s at ABC?

Putting F5 in front of other network/security devices are usually no problem at all and I have not seen anywhere stating that you cannot put an F5 device in front of Citrix Netscaler. But with that said, since you have a complete new set up IP addresses, I'd rather just set up a replica of the application within the Citrix NetScaler environment and make sure that, load-balancing, iRules, WAF works completely, before making the move to NetScaler. When you're ready to move the application you can simply just change the DNS to point to the new NetScaler environment. If something fails you just change back the DNS to its original configuration.

Why do you still need the F5 to be involved?

2. At a later stage how we can migrate the F5 configuration at ABC to Citrix Netscalers at XYZ? Are there any tools to migrate iRules, WAF settings etc.? Much appreciate your reply.

There are supposedly a conversion tool which the Citrix NetScaler SE's have. But the experience from other customers is that it's usually not that clean and might introduce weird problems. The suggestions state that you should set it up manually. Since you have iRules, converting those automatically is supposedly not a great idea. Check out this thread from the NetScaler forum: https://discussions.citrix.com/topic/235530-f5-to-netscaler-migration/

But if it's still interesting, check with a Citrix SE. :)