It is recommended to create a new wallet but what about old wallets? Will we be able to get rewards for new wallets?

you can claim in v2 of the app which is now live on the play store and the App Store. Please read this medium article fully which explains everything https://legionnetwork.medium.com/legion-super-app-v2-launches-be4bea482a27

Yes you will get your rewards You can also import your previous wallet.

yes create new wallet and confirm it on profile>>claim rewards

You can get your rewards in new wallet