Only one change since v4.4.34, however an important one for some use cases.
Do not clear IPs from DHCP WAN interface in PREINIT of a forced renewal. The primary problem symptom is when the system is provisioned by the controller when booting up (most always only the first boot post-upgrade).
In all firmware versions prior to this, that provisioning would remove the WAN IP then go through the DHCP lease process. This would leave a single WAN system with no Internet connectivity for a few seconds at least, and in multi-WAN cases, would cause a failover and fail back. In most cases, that was the only noticable symptom and it self-recovered without any problems, but in some it could cause issues that require another reboot post-upgrade. In multi-WAN cases, it could cause WAN to stay in a down state on the first bootup post-upgrade. In normal operation of the system, there are no forced DHCP renewals (this isn't relevant to the normal process of DHCP renewal).