Database upgrades

Is there a tool that would let me upgrade a database with a new
schema (without loosing data)?

I have a database, x weeks later there is a new schema.. I want the
possibility to skip that upgrade (if I don't need the added features,
I don't want to upgrade)... When I decide I want to upgrade (I need
the added features or whatever) I should be able to do so without
loosing data...

The schema is 'complete' in each upgrade...


Do I make any sense?

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