i can keep is that way or do i need to download the new wallet, in order to not lose my coins.
The best solution to keep your Stratis save, is the hardware wallet.
For only 70 euro you can buy the Nano Ledger S.
You can store 15 different coins on it.
This is how I keep my money save.
Just take a look at www.ledgerwallet.com/products/ledger-nano-s
IMO he can store Stratis on bittrex, poloniex or binance, these 3 exchanges are quite safe and secure. I store my coins on bittrex since 2014. Never have any problem.
Just because you've done it successfully doesn't mean it is good practice. Tell that to the people that had coins on Mt. Gox, Bitfinex, Mintpal, Cryptsy and countless other exchanges...
if you don't staking ledger nano S is well!
else use strat wallet on pc and safe your backup file!
exchanges are well but don't deposit all of your coins here.
as trader you must use exchanges but is not best practice to hold lifetime