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There are only minor changes from bbPress 0.9 templates to 1.0 templates – you will just not have a few minor features that are new in 1.0 with a bbPress 0.9 template. You might have to make only a couple of minor adjustments after upgrading.

However the reverse is not true – a 1.0 template will not work with 0.9 – it breaks a great deal of backward compatibility.