Here are some of the era combinations:
Pre-DGSD era (Pre-30 years, when Xiao Yuanshan's family got massacred) and DGSD era.
Pre-LOCH era (First Huashan Tournament) and LOCH era.
end of LOCH era and beginning of ROCH era.
Middle of HSDS era (after all of the ROCH Greats died; 40+ year old Zhang Sanfeng) and developed HSDS era (100+ year old Zhang Sanfeng)
and others./
Which era-to-era change represents the biggest gap? Here is what I think:
Pre-DGSD era and DGSD era: most of the assaulters of Xiao Yuanshan were considered First Class Fighters of the era. Reverend Zhiguang implied that Pre-30 years Zhao Qiansun would be a First Class Fighter. Now, pre-30 years Zhao Qiansun could not even take one stance from Xiao Yuanshan, and present-day Zhao Qiansun got knocked down by Qiao Feng in one stance. Pre-30 years Zhao Qiansun was probably at 50% of present-day Zhao Qiansun's level, which is probably around 50% to 75% of a present-day First Class Fighter's level (Ding Chunqiu, Murong Fu). So, pre-30 years First Class Fighter's level was roughly at 25% to 35% of present-day First Class Fighter's level. Also, Zhiguang said that Xiao Yuanshan was the absolute best under heaven 30 years ago. Xiao Yuanshan probably increased his MA by 2 fold during the next 30 years. Summary: Pre-DGSD era MA standard was about 30% of present-day DGSD standard.
Pre-LOCH to LOCH: Well, it was hard for Dugu Qiubai to find an opponent of Ren Woxing's calibre. Some of the Greats' MA had not been fully developed yet. So, Pre-LOCH era probably =~ 50% of LOCH era.
LOCH era to ROCH era - probably just a small improvement, like 10% to 20%.
Middle of HSDS to End of HSDS - this should be a big gap; Zhang Sanfeng was undefeated throughout his life, which means even when he was young he was probably undefeated/unmatched. Also Shaolin saw a huge improvement over the ROCH era.