Freeciv 3.0 comes with a utility called freeciv-ruleup. It can be used to update ruleset from the format of the previous freeciv version to current one. The output is not pretty, but it should be loadable to freeciv server.
Freeciv-ruleup loads a ruleset given with --ruleset commandline option, or the default ruleset name of the version ruleup itself is from (not the version it updates from). You should set environment variable FREECIV_DATA_PATH to point to old version rulesets directory prior to running freeciv-ruleup, so it finds those old rulesets instead of the ones matching its own version. Once ruleset has been loaded and converted to match current freeciv version, it's saved to a directory named '<ruleset>.ruleup'