Some of Fireboxes are about to have licenses expired. The only features we use are multi-wan and VPNs. Will the expired license create any problems to multi-wan and VPN function?
No it shouldn't.
All functions which are not security add-on licenses should continue to work as before the expiration.
If you haven't done so already, you should upgrade them to the latest Fireware version available for each firewall.
No you normally can't.
There was a recent exception as a result of the cyclops blink exposure, where current firewall models could get upgraded even if there was no active support license.
"All active Standard and Gold Support subscriptions include phone and web-based support, software updates and enhancements, and advance hardware replacement (RMA)."https://www.watchguard.com/wgrd-support/support-levels/terms-conditions
Without a support license, none of these are available for a firewall.
i have a question about firmware since you mentioned it. You talked about upgrading firmware to the latest version. When the license expires, I cannot upgrade firmware anymore?
With an expired license, you can't upgrade the Fireware version on the firewall.
You can find more information here:https://www.watchguard.com/wgrd-support-services/overview
The firewall will still continue to run, and work on the version it is currently running.
WatchGuard Customer Support