TomatoUSB recommends a NVRAM reset (or 30/30/30 reset) before and after upgrades.
This means you loose all your settings which causes a lot of people to not upgrade at all.
The steps to export/import are a bit vague as they depend on what you want to save.
It basically comes down to do this on the old configuration
Save that output to a local file and then use a search tool searching for specific sections you want to restore.
After you restored the sections ensure you persist them:
nvram commit
This is what the TomatoUSB author usually searches for: