opkg upgrading packages is possible in theory, but can/will create subtle bugs & incompatibilities, if you try upgrading "core" packages.
Upgrading vanilla non-core packages may work, but trying to upgrade system packages like procd, ubus, ubox, busybox, etc. may lead into major trouble if you end up with incompatible versions. (in the worst case, wrong upgrade order breaks your opkg & sysupgrade capabilities)
So, unless you really know the package to be upgraded to be a harmless non-core package, do not opkg upgrade it.
If you update the main firmware that often, there is usually no practical reason to opkg upgrade packages in meanwhile.