your nxt wallet should contain nxt and your ardor wallet should contain ardor and ignis if you had the nxt before the snapshot. The same passphrase will work for both.
If you delete either, you are just deleting the client, but usually the database (this is the big file you have to download with the client, or can start with a part database from around the fork time from jelurida here you would have to unzip it and replace the old database. Also noticed it labelled Ardor-nxt_db but should be named just nxt-db) you can access the database file by typing in your file explorer
then click on ardor folder. You can see the file size, so the larger the file size the more recent the copy of the database.
If you delete everything and start again, you do not lose anything. If you have your passphrase it will work on any ardor/nxt client. It will usually just overright your old nxt_db file if you dont do anything. Label the old database nxt_db_old and then you can load the new next client the stop it. Next delete or rename new db file and rename old file to nxt_db. This will keep your old db if you dont want to download again.
I just downloaded the windows installer but gave my mobile-wallet version. Almost instant.
Then I deleted it and downloaded the universal and it took about 30 mins to download the entire blockchain db using a 4mbps internet connection.
Nxt had a feature to download the latest version of java but ardor didnt say anything when I didnt have jave. After getting jave worked a treat.
Hope this helps