I'd suggest you push back to the client for the reasons I said above.
When sites are updated, for example from 6.3 up to 6.20, there are often database updates. These are one way, so whilst you could try to reverse them, you are doing something that is not supposed to happen and hence not documented. Also, like I said before, you'd be losing the benefits of the bug fixes.
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Why would you want to do
Why would you want to do this?!
Drupal 6.20 and 6.3 contain in the whole the same features, but 6.20 has the advantage of bug fixes including security fixes.
In addition, apart from perhaps a couple of edge cases, all contrib modules that work with 6.3 should also work with 6.20.
Therefore, my advice would be to stick with 6.20!
To meet the requirement
Thanks for your suggestions.Actually client requirement is to have 6.3.
So any document or steps for downgrading available?
I'd suggest you push back to
I'd suggest you push back to the client for the reasons I said above.
When sites are updated, for example from 6.3 up to 6.20, there are often database updates. These are one way, so whilst you could try to reverse them, you are doing something that is not supposed to happen and hence not documented. Also, like I said before, you'd be losing the benefits of the bug fixes.