"whereas previously" is correct and can be used in written English.
You could use it to compare two points that are different from each other. For example: "Whereas previously we had to send letters in the mail to communicate, now we can talk to each other instantly via text messages."
Whereas previously a member of the top-16 could make a decent living in losing prize money without actually potting a ball, now he must win matches to make money.
Another told investigators that "whereas previously it had been easy to carry stuff over into the border into China, now a few people have dropped out of the business completely".
Whereas previously you may have been relying on key consumption times when people were able to be sat at their computer, now you have a consumer who you can reach with your product all through the day.
Whereas, previously, Kurdish movements had sought the creation of their own state, most have tempered demands now to a degree of autonomy similar to that in Iraqi Kurdistan.
Whereas, previously, they were used to transport industrial produce, most of them are now used as reservoirs, either for domestic purposes or for powering hydro-electric schemes.
Whereas previously a user would have to wait until they got back to their desk to open up Do in the browser, they can now talk to the Do app whenever a relevant thought comes up.
Whereas previously described dual-incision techniques have focused on posteromedial and posterolateral approaches to the ankle, the authors' posterior exposure, in particular, uses a midline skin incision and a longitudinal split of the Achilles tendon.
Whereas previously our requests would go unheard, now we can expect requested books to arrive within three months".
Whereas previously, winning the argument was the absolute dog's bollocks.
You could almost call them clay-based poems, whereas previously they've been written on millstone grit.
Suddenly there was focus and intensity whereas previously there had been complacency.
