I have not been able to find a way to load tax rates in CE 1.9 that are effective from a future date, or to enter increase/decrease in tax rate at a future date. There appear to be no tax validity periods anywhere. Can anyone help please.
Magento's tax rules can be modified in realtime(and re-indexed), but you cannot set it in a date validation way. So you cannot add future taxes.
You have to take a note of the global tax setting before change anything, if you enter prices including tax and you change your tax, after changing the price excluding tax will be lower if the tax is higher.
The other way around, if you change the tax setting and enter prices are entered excluding tax, it can happen you physical prices(like 1.99) will be screwed after your change.
That's all I can tell for default tax settings.
Since this now became quite relevant for German shop owners, we implemented CLI commands for M1 and M2, which let you change the rates of your tax rates. This way, you can set up a cron job, which changes your rates at a specific point in time. There is a blog post explaining the complete process, but in the end, it can be a cron job as easy as this:
0 0 1 7 * cd /path/to/your/shop && n98-magerun.phar tax:rates:edit --ids 1,2 --rate 16.00 --update-titles