Theoretically you should be able to import a config from any previous pfSense version. The upgrade scripts are cumulative so it should be updated to a current version complete with all the required chnages when you do.
It's a relatively easy test. You can roll back to your current config from the console if it fails for some reason.
If it fails yo boot to the console re-installing entirely is quite fast if you have the install media to hand. Particularly if you put the current config onto it so it boots up ready to go first time:
If you WAN is wireless you should make sure you have tuned the monitoring to match that. It will almost certainly have higher layency and packet loss rates than other WAN types. You might just disable the WAN monitoring action for that gateway to be sure it's not triggering unnecessarily.