I am writing custom addons for my company. Over their life these
addons will accumulate data in the tables I have modelled, using the
orm.

During development, when I want to update the schema I have to
uninstall and reinstall the addon. It will drop all the tables and
recreate them.

The question is, when I am deploying a new version of an addon to
product (because it has new features which need new table columns,
etc), how do I safely preserve and migrate the production data to the
new schema without losing it?

Edit:

I can't comment on Baiju's answer because it requires 50 karma. Anyway Baiju, thanks. I take your answer to mean that Odoo will automatically add new tables to the database based on any new fields added when I upgrade a module. I will make sure to back it up just to be safe, but this was the answer I was looking for.