We'll never know for sure. He didn't specify what he meant, the constitution itself or any of the amendments. The constitution is fine. Amendments are for repealing or introducing ideas.
His personal position and his ideas and theories on them varied a bit. He was firmly against the institution of slavery, but owned about 600 slaves. He advocated some sort of manumission program for eventually freeing slaves but did little more than suggest that process. He did not want to remarry after his wife died, but kept a slave/concubine and fathered 6 children with her. He felt that all laws, contracts, the constitution, etc. should expire in 19 years, and the 20th year was for rewriting them, but did nothing more than to suggest that. Brilliant people have silly ideas from time to time, just like dorks have good ideas every now and then as well.
Jefferson didn't pretend to know what the US would be like in twenty years, leave alone 200, so he wasn't specific about what changes would be needed. He advocated a process that would allow us to change the government to meet the needs of the people of that future date.
I agree with that sentiment, but I also feel that's what the amendments were meant for. Just because they (gubment folks) don't use it properly doesn't mean it (the constitution) should be trashed. Think of what silly stuff the millennials would put in there when it came time for rewriting. We'd be stuck with Sharia Law, no rules, no borders, free stuff for everyone, etc.
The one quote of T.Jefferson that always makes me pause,especially in times like this is,"the tree of Liberty must be refreshed from time to time with the blood of patriots and tyrants.it is its natural manure."
Well, he was a little off...We had another rebellion in less than 90 years, rather than his 150. Not to speak of his "a few lives lost"
We've had 27 amendment ratified, IIRC. That's just over one per ten years. The Constitution hasn't kept up with the United States.
It seems to working out alright the way it is. Always room for more rules, regulations and laws though.