If you install the newer version, it should default to using the same wallet file (with all the keys, transactions etc) as the previous version... assuming you had your wallet file(s) stored in the default appdata location.

However, if you had your wallet file stored in a non-standard location, you may need to go "file->open wallet" and navigate to where the wallet file is located.

In either case, the newer version should be able to read the wallet file without you having to do any conversion or export etc.

Note: even the latest version of MultiBit Classic will only allow a max fee of 50,000 satoshis which could lead to your transactions getting "stuck" due to low fees.