You can upgrade directly to V3
the download site has an 'archives' link to older firmware.
that said, I am not convinced you need to do this to fix a VPN issue.
I cant remember back that far, but there should be a 'status' ( wont be called this though ) link available on the ipsec page for each tunnel....this would be a starting point to diagnosis.
After I posted I did some more digging and found the firmware download page. I used the latest available netflash utility to up the firmware to a 3.x version, then flashed the current firmware via the upload process after that. In the end, it was uneccesary, but I feel more comfortable having current firmware loaded on the device anyway (at the very least, the new firmware gives me the option to toggle PFS on IPsec tunnels, which just defaulted to always on in the older one.
The problem, amusingly enough, was a single checkbox on the Linksys VPN router at the other end. It had been reset, and when the guy tried to restore the settings from a 5-year-old printout he selected a box he shouldn't have. Just goes to show that you should never believe people when they claim "everything is the same as it was"
yes, I have heard that many many times.
part of the challenge, is tro assume nothing, and inspect youself