Upgrade cluster from 6.8 to 7.x


I am about to upgrade some clusters from 6.8.4 to 7.3.2 (7.4.2 soon) and I am not sure if I should reindex all of my indices in these clusters.

In documentation I see a lot of mentions about the indices should be readable. But when I have indices created in 6.8.x will I be able to write to them in 7.3.x?

It's really hard to me to find out more info about indices versions compatiblity - I mean more than indices from 5.x should be reindexed if i want to upgrade to 7.x cluster.

Thx for sharing any related info with me.