When you are about to order an upgrade via Customer Control Panel (CCP) you will usually be informed what upgrade paths are possible and the costs resulting from it. There should also be a hint directly answering your question.
Data will usually remain in place, if you choose an upgrade that can be performed automatically, but making a backup is always most adviseable, though.
If you booked another, additional tariff, that's no upgrade, but a migration. You will have to copy all the data from hostingX to hostingY, then.
For the migration of emails there is a migration tool: https://mailsync.netcup.net/de/
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