A country doesn't have to leave the Commonwealth just because it becomes a republic. 33 of the countries in the Commonwealth are republics. Including India.
I like the idea of a commonwealth. It's hereditary monarchies I'm not so keen on.
The Commonwealth is actually quite an interesting entity these days, I think the previous most recent joiner was Rwanda, which had no links with the UK colonially. They get something out of it more than a Royal connection.