In fact, that is exactly the correct way to upgrade your device, and has been for as long as OpenWrt has existed. Due to the shortcomings in the opkg package manager (and some still lingering in snapshot's apk), it has never been possible to safely upgrade arbitrary packages.
Please see the various support threads for the ASU tools:
LuCI Attended Sysupgrade support thread
The OpenWrt Firmware Selector
Owut: OpenWrt Upgrade Tool