「今から」 means "from now on" and 「これから」 means "from here on" or "from here". So, これから actually has 2 meanings.
So are the "from here on" meaning of 「これから」 and "from now on" meaning of 「今から」always interchangeable in Japanese? In english I guess they almost everywhere are but "from now on" is used more because "from here on" can also be used to signify place.
Please provide examples if they are different in usage.