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From my experience, AOS should interpret an older startup config from an older software version into the new version upon startup after the upgrade. But you definitely want to have it go through startup and not try to copy and paste an old config into the running config on a 3448 running a newer AOS. There are definite changes in configuration language from an older version like 18.x and the newest revision train of A11.x. That said, it may not affect you depending on what your configuration looks like. Unfortunately you would need to go through all the release notes of just about every revision to look for changes to the software that could affect your configuration.
What I like to do in a scenario like this is take a working config, load it on a spare/test router, perform the OS upgrade and ensure startup occurs correctly and that important configuration points are maintained. Hopefully that is an option for you.
Thanks for the reply! I sadly don't have a spare, but some of these offices are only open a handful of days during the week so I should be able to test it at one and have the day to figure it out if there are huge differences in the config.
You can upgrade easily as the configuration will be re-written when the device boots. The problem is if you want to downgrade. The biggest change is that most AOS devices are now dual-stack IPv4 and IPv6. The keyword "ip" is added to a number of commands. You may also find ipv6 unicast-routing enabled by default and a few new features that you may or not want.
I would recommend storing the existing running configuration to a file, doing the upgrade, and running a diff on the configuration after the upgrade. RANCID is a great free tool that will handle this automatically as well as backup all of your configurations on a schedule. Should you ever need to replace hardware in the field, you'll have the latest configuration on file.