What are the cons for downgrade ecto version in phoenix app?

I am new to elixir, I installed latest version of phoenix, i would like to use mongodb in my app, but mongodb adapter not supports current version of phoenix_ecto. so i am planning to downgrade the ecto version as given in this link. Is there any disadvantage in downgrading ecto?

You wouldn’t get some new features probably. You can see which in the changelog [0].

btw there is an issued on supporting ecto 2.1 [1]

[0] https://github.com/elixir-ecto/ecto/blob/v2.1/CHANGELOG.md
[1] https://github.com/michalmuskala/mongodb_ecto/issues/109

Later can i upgrade ecto version once i get support for current version of phoenix_ecto?

What do you mean? I think phoenix_ecto already has ecto 2.1 support – it’s mongodb_ecto that doesn’t. But yes, you can upgrade later just by changing deps function in your mix.exs and running mix deps.update ecto.