I honestly don't think is possible to update de UDRs via APPI calls in the new template, since the cluster-ha works completely different to the previous one. Also, the failover times are lower in the new template, since you are using the same IP in the UDRs (the backend balancer) and no changes are done (depending of the numbers of UDRs it could time some time in the previous template)
Also for the migration keep in mind:
-Possible Changes in NAT policy Rule for the Cluster. This is because the previous template used NAT RULES in the FrontEnd Balancer, while the new one uses Load Balancing Rules (so you have to migrate all the NAT rules configured in the FronEnd balancer to LoadBalancing rules in azure and also make changes in the NAT policy Rule in the smartconsole.
- Public IP addresses. I don't remember if the old template uses standard or basic Public IP address, but the balancers in the new template are the Standard kind, which means that the Front End Balancer can only use Standard Public IP Addresses (in case you want to reuse the Public IP address from the previous cluster for publications)
-Do you have any VPN site to site? Only in the latest R80.40 template (actually, maybe since the October R80.40 template) you can reuse the Virtual Public IP address for the VPNs site to site, but you need to make some changes. Keep in mind that this IP is different to the one used for the FrontEnd balancer.